Unstoppable Overview

Ultra Mutant is the rarest achievement because of the time needed to get all the unlocks, however Unstoppable as the second rarest may be the most difficult overall. This difficulty can be offset by following this strategy guide which will give advice on how to handle various parts of the game.

The specifics of this achievement is actually simple. Start a run as Melting and reach the Labs, taking a lethal amount of damage in a Necromancer revive circle. This will transform Melting into the secret character Skeleton, who cannot be unlocked. While he has 4 max HP to start with he suffers from poor accuracy and speed making him the most difficult character. Skeleton will be reset to level 1 and the achievement pertains reaching level ultra, which takes a total of 2,701 rads.

Transforming into Skeleton will carry over current weapons/crown, but starting over from level 1 with no mutations right before the Palace & Throne fights will make death very common. Being in a constant vulnerable state means that repeated failures may be a staple of early attempts.

Skeleton has differing mechanics dependent on version:

Generally Non-Beta is recommended because of the Hammerhead bug to help get Unstoppable, while Beta is the better option for meta runs because of rad weapons. If trying to get ultra for the first time then leaning on rad weapons is suggested against. Bloodlust + Ultra Crossbow is a broken build even before getting ultra on the betas but some familiarity on Skeleton and playing ranged may be needed first.

Setup Phase (Melting)

Skeleton starts at level 1 with 0 mutations and only keeps held weapons/crown. So while Melting's main goal is to just get to Labs, some important setup can be done to help set up the success for a run.

The first manner of business is weapons. Melting should take weapons that not only helps him get to labs but that Skeleton can also get through the Palace and Throne fights with, and may even have to fight Elite IDPD if there are no suitable replacement drops by loop. Skeleton will struggle with any very high ammo consumption weapons like most bullet weapons, as his slow speed may make him miss out on ammo drops and have more risk trying to collect them. His inaccuracy will hurt weapons with wide spreads such at pellet type(shotgun) weapons, and some bolts. Melee weapons can heavily help offset bad ammo or dodging mobility, though some of them can fall off by loop once dealing with the aggressive Elite IDPD.
Taking a weapon to clear Freaks with such as Hyper Rifle can be useful as sometimse Labs has overwhelming spawns. Otherwise for the upcoming Palace and Throne fights Skeleton would want to look for any long-range weapon that can deal good damage while keeping Skeleton out of risk. Hyper Slugger, Grenade Rifle, Heavy Crossbow and Assault Slugger are some commonly seen weapons that deals a good amount of damage without eating up ammo at a solid range. Plasma Cannon, Gatling Slugger and Hyper Launcher are all great damage dealers while eating ammo much more quickly. (Be mindful with explosive weapons and teleporting Guardians).
Jackhammer is great for consistent defense with many projectiles to worry about as a constant defense. Energy Sword is very viable given the right mutations, but can struggle with an unleveled Skeleton.

Mutations can be taken to either better Melting's chances of reaching Labs, or helping increase Skeleton's chance of survival.

The sets that Skeleton sees will (mostly) be the same mutation sets that Melting did, meaning that the player can plan what mutations Skeleton can build around. However there are multiple factors that can effect what sets Melting and Skeleton actually sees which may lead to confusing results, there is a whole section on the Daily mechanics page(link later) that explains everything. This is not required reading, however it explains why Skeleton sets may differ from Melting's.

The Daily guide can also be utilized with the Scouting Sheet. While the Daily can only be tried once if other people have scouted that information can be utilized to give that one run its best chances of success. If the Weekly challenge happens to be Melting it can be played repeatedly until level ultra is reached, though starting crown can make or break this experience.

Current crown is also transferred over to Skeleton, however most of them are particularly unhelpful because of his frail start. The only recommended one to skeleturn with is Crown of Curses as it has a slightly better drop pool to pull from, and cursed weapons can be uncursed upon looping/Desert vault.

Skeleturn Strategies

When going for Unstoppable the best option is to transform in preloop labs. Necromancers additionally will spawn in the Frozen City during loop, but is not a safer alternative. Necromancers are not guaranteed to spawn in preloop Labs, so sometimes there may just be bad rng.

Melting must take a lethal amount of damage within a Necromancer circle in order to become Skeleton. Strong Spirit will block lethal damage so it must be dropped before turning. Necromancers will attempt to revive the corpse of any killed enemy or prop that is within their line of sight. This is not a frame perfect trick, as long as the circle is active Melting can skeleturn within it.

Lethal damage can include any damage source including enemies or self-inflicted.
For self-inflicted, this can include weapons that cause explosive/toxic/blood damage to Melting. Some of these weapons may be difficult to time properly and are not ideal, and Skeleton will likely have to use that weapon through the Palace. The best weapons at this point would be Grenade Rifle or Hyper Launcher, as both are great weapons to handle the Palace/Throne fights, and if a Necromancer circle is by a wall then Melting can transform with ease. Blood Hammer is also a good pick but can be riskier to take to the Throne fights.
As for enemy sources, the safest way to turn is the Freak method below, compared to Rhino or Explo Freaks who will actively walk towards the player even if line of sight has yet to be made.

The Freak method is recommended as setup can be relatively risk-free once understood. Freaks have two modes - an 'idle' mode where they will not move from their position and an 'active' mode where they will run around the level bumping into walls and the player. Their active mode is permanently entered as soon as the player enters their line of sight, but revived Freaks start in idle mode.
Using this tech, if a group of Freaks is killed near a corner and a Necromancer respawns Freaks from the back of the cluster first, then once they revive the circle in front the player can quickly turn the corner running into the circle to become Skeleton. As Necromancers have a tendancy to walk away from the player, getting a proper setup can sometimes be a bit time consuming.

As shown in this image, the Freaks at the bottom are in an idle mode. If Melting rounds the corner and runs towards the bottom of the circle, he will be damaged by the Freaks instantly. Make sure Melting is at 3 HP or below without Strong Spirit active.

Necromancers are fairly fragile. Randomly shooting down hallways or approaching corners where they are clustered with Explo-Freaks can cause them to die which may cost that attempt. The best way is to take Labs slow and react where necessary. Melting's active should only be used sparingly to avoid accidentally killing Necromancers as well, only used when needed to clear out excess enemies if they are getting repeatedly revived in a way that makes it too dangerous to try to skeleturn.

Necromancers cannot damage Melting directly so if any get in a corner with no enemies in their line of sight, Melting can simply walk behind them and push them towards an area where they will start to revive enemies.

Skeleton Skillset

The only upside of Skeleton is Blood Gambling, as he is the only character in the game that does not have a beneficial passive. Blood Gambling allows Skeleton to shoot a weapon without consuming any ammunition, however there is a chance that it will damage him for 1 health. Skeleton's character page has the full details of the tech behind it, but essentially the more ammo one shot uses the more likely it is to damage Skeleton, in relation to how much ammo a pickup of that weapon type gives.

Each negative passive ability limits Skeleton in different ways;

The other best mutation is Bloodlust, if taken then Skeleton can blood gamble far more frequently which saves on ammo. Any mutation that is generally good holds true on Skeleton such as Bolt Marrow, Long Arms, Trigger Fingers and Laser Brain while not as high priority. Euphoria and Hammerhead are decent picks here too if playing it safe. Thronebutt can be taken if a high ammo consumption weapon is found such as the Plasma Cannons or Super Flak, being paired with Bloodlust and Trigs makes Skeleton pretty powerful from the getgo.

One of the best builds in loop will always be double energy which includes Energy Sword and Plasma Cannon or Super Plasma Cannon. This will need Long Arms + Laser Brain, and SPC will also demand Trigger Fingers. Ammo drops are guaranteed to always give energy while not at full ammo, where the Plasma Cannon can deal with clearing out areas, Energy Sword focuses on nearby threats and Elite IDPD. Caution should be used when IDPD spawns in because shields can deflect any of the larger plasma balls. Focusing on a bolt-based build can be effective as well in terms of handling threats in a constant fragile state.

Post-Labs Gameplay

The Palace

This is easily the most dangerous area to start out with. Depending how Labs went Skeleton may still just be Level 1 without mutations, and will likely not be at full health yet. While starting in a fragile state entering 7-1 has a good chance of IDPD to spawn all around as Palace enemies close in around spawn giving Skeleton very little breathing room.

Leaving spawn can be dangerous depending on the enemies around, so camping around corners is a safe way to start. Dog Guardians will pounce at Skeleton when within range and can easily break walls surrounding spawn, so they take high priority as well as IDPD who can chuck grenades. As for Explo Guardians & Normal Guardians, they're less of an immediate threat and can be held off on being killed until they hover/teleport close to spawn, so their pickup and rads can be collected easier, however bear in mind any guardian enemy killed will break any walls immediately next to them.

Icon:
Melee: 6 DMG 2 DMG 2 DMG ~ ~ ~
Projectile: 5 DMG 2 DMG ~ 3 DMG 5 DMG 3 DMG
Priority: Low Medium High Medium High Medium
Notes: Frequently will teleport if in line of sight. Not a big threat so long as there is cover. Easily causes chip damage and can be an annoyance if hiding behind Dogs. Highly aggressive nature towards the player. Usually easy to take out, only an issue if swarmed at spawn. Telekenesis and a one shot projectile is a death sentence for Skeleton's slow speed. Can shoot from off-screen and deflecting is annoying, making leaving spawn more difficult.

Priority is a general assessment on what will give Skeleton the toughest time, as Dog Guardians and Inspectors can both one shot Skeleton with aggressive attacks they're threats that need to be handled as soon as possible. If a melee swing is used on a Dog Guardian while it jumps the melee will ignore enemy iframes doing massive damage, though it puts Skeleton at risk needing to react quickly. If not running melee then Dog Guardians can be handled by hiding around corners and sniping, at the risk they may pounce and break cover. If out in the open trying to strafe along the walls away from the Dogs. This may let Skeleton barely avoid getting hit but is the least desirable situation to be in.

The main reason the Hammerhead bug is suggested to be abused on Non-Beta is because Dog Guardians are very aggressive enemies but can't fit into 1 tile gaps and can't break walls as quickly as Skeleton can dig in, and the immediate spawn with an IDPD swarm can sometimes instantly end a run for Skeleton. Breaking a few tiles on 7-1 and 7-2 makes for a far safer defense.

Another issue in Palace is retrieving pickups as most enemies have an aggressive nature and so collecting anything can come at a risk, which is why it's best to kill enemies closer to spawn for pickup and rad collection. Good mutations in the first few sets can be vital to Skeleton's survivability here. While blood gamble is useful to reduce ammo consumption, health is also going to be in short supply for these first few areas. It's best to only use the active upon reaching loop or having a healing mutation early on.

Leveling in these early areas depend entirely on how well spawn looks. If Skeleton needs to keep retreating further back then he may barely level at all, but a spaced out Palace area will let him level much easier. That is why taking it slow and being methodical about clearing out enemies is the best way to safely level, while also taking more time per attempt.

The Throne

The Nuclear Throne is very intimidating as two of its three attacks can oneshot 4 HP Skeleton, being the laser beam and orbs. The normal projectile bullets will only deal 2 damage, but being hit twice in quick succession is common. It's common for melee to be used here as if the Throne cycles through too many spam-y attacks Skeleton can easily find himself in a situation he can't get out of.

Similar to normal Throne strategies, the secret here is to stand on the red carpet the entire time. The Throne will give a very loud audio cue on when it will use its laser attack, and during that time it cannot use its other attacks. Simply sidestep the laser then return to the carpet once the attack is over, which the Throne may decide to repeat that attack. It's easy to focus on destryoing the generators then attacking the Throne when its mainly using the laser beam attack, so the fight can be a bit based on RNG.

Generators drop rads/pickups but if walking over to collect them the Throne may cycle through its two less desirable attacks, so the safest option is to run over the moment it starts its laser beam attack. Though if not running Plutonium Hunger/Extra Feet yet or melee to counter projectiles, leaving the red carpet for too long will be high risk.

Guardians activated by the Throne should be taken out swiftly to avoid extra projectiles Skeleton has to worry about. This fight is mostly about good reaction time, so as long as the Throne beam attack is continually baited out Skeleton should be able to overtake this boss.

If Skeleton already has good weapons/mutations and wants to play it risky, after the Throne makes its half health noise(destroying generators) and its low health noise, he can do a bit of extra damage wihle not completely killing the Throne while running to the bottom of the level. Once all the Guardians are awakened by the Throne he can quickly kill the boss and will gain a bunch of extra rads from each Guardian insta-killed, but if the Throne's health is misjudged this can lead to a quicker death instead. However, an extra level can go a long way for the next boss fight.

Whatever the 7-3 weapon chest is Skeleton should bring it to 0-1 with him, if it's cursed he can quickly pick it up before entering the loop portal in order to uncurse it. Bringing it may come in handy for the Throne II fight, but also to deal with the 1-1 L1 spawn.

Throne II

Entering 0-1 L1 will have multiple IDPD right away, Skeleton should immediately locate the best corner while spawnkilling any IDPD he sees along the way. Hugging a corner to hide in case of multiple Shielders or Inspectors is the safest way to go about it.

This boss can be another skill check for Skeleton, as it is a fight where slow movement is going to be especially felt. The sooner the fight is ended, the better, as Throne II can spawn laser projectiles that deal 2 damage each and can easily overwhelm/corner Skeleton, on top of the normal orb attack that can one-shot him without Rhino Skin.

Melee can be helpful here as well to carve through any projectiles on screen, else trying to circle around the center of the arena while tailing Throne II around, chipping away at its health is the best Skeleton can manage. All the while staying on high alert for laser attacks that need to be sidestepped quickly.

Skeleton can also use the extra weapon from 7-3 to quickly pick it up, fire it, then swap back to his main weapon and fire that as well. Hotswapping resets weapon cooldowns for faster DPS but at the cost of standing still while doing it, so should be done the moment the fight starts before projectiles spawn.

The IDPD

The entire time Skeleton is in Loop he will have to deal with Elite IDPD. They are an oppressive force normally and is far worse for Skeleton to have to deal with. Where Palace and The Throne fights are so difficult because Skeleton starts in such a weakened state, throughout loop there are multiple enemies that can instantly corner Skeleton and bring an end to the run.

Elite IDPD will punish different kinds of playstyle, so depending on what spawns Skeleton may have to react differently. IDPD spawns that have Elite units have a different audio cue than the normal one, so pay some attention to the noises.

Vans are usually easy to dodge if audio cues are paid attention to even while playing as Skeleton. Their spawning condition is based on a random percentage of the level cleared, with the condition of only spawning if any IDPD have been killed during that level. The best way to handle Vans is to either have Skeleton destroy them before they unload IDPD if he has the proper DPS, or to immediately fire upon the back of the Van the moment it stops and unloads IDPD. Stationary Vans can be a bit of an issue if Inspectors or Shielders use their bulky HP and size as cover, as Skeleton struggles with pushing.

The Desert

Desert spawn can vary heavily on spawning as well. Flies do not drop ammo/health making them a bit of an ammo sink and must be dealt with first because of their aggressive nature, so an open spawn with a lot of flies can be an immediate problem. All the while enemies are trying to push into Skeleton's spawn, Snipers and Scorpions may be shooting projectiles from nearby corners and may be just out of reach, which makes pushing out of spawn even more difficult for Skeleton.

While the Palace/Thrones can be scraped by with bad weapons/mutations, Skeleton is going to feel how bad Desert is fast, especially once IDPD starts spawning during levels. That is why keeping an eye out for good mutations in early sets from Melting and finding good weapons is vital, usually at Desert any normal mutant would've had a mostly or fully complete mutation set already. Running just single target weapons will hurt immediately with Flies/Maggots invading spawn, while running just short range weapons will hurt with clustered Scorpions/Golden Scorpions as slower speed means difficulty pushing and difficulty running away if there are too many projectiles.

If Skeleton has Bloodlust at this point he can lean more into using blood gamble, especially if running any weapon with a high ammo cost. Any enemy that procs Bloodlust is essentially another free shot. If running Rhino Skin, keep in mind that no enemy does 6 or 7 damage inbetween the Desert and Caves(with Dog Guardians next appearing in loop Frozen City), so Skeleton can always gamble twice without being at instant risk of being one shotted because of it.

Upon reaching the loop vault Skeleton can choose between any of the 6 crowns, for the sake of this achievement Blood can be ignored.

In other words these crowns are dependent on what Skeleton's current build already is. Crown of Love is usually the safest crown to run, the other crowns are only good to run if the current weapons/mutations already allow for them to be viable.

Entering 1-3 L1, Big Bandits are similar to Dog Guardians that any melee weapon can hit through iframes on charge attacks. Otherwise, they should be taken down quickly but generally just two of them are manageable. The important part is to not get cornered as multiple hits in a row is a game over for Skeleton.

Post-Desert

For the Sewers, be wary as toxic clouds and toxic Ballguys can overwhelm Skeleton. Mom should either be pushed before IDPD spawns or after they're taken care of if possible, depending on the layout of the level. Her tanky HP is one of the last big roadblocks, so quickly swapping to a different ammo type from any weapon on the ground for extra damage can speed the fight up.

If Skeleton has been able to collect enough rads inbetween all the chaos he should be decently leveled enough entering the Sewers, and hopefully has weapons that can manage. The biggest threats on these areas forward will continue to be Elite IDPD, though Buff Gators will be good at catching Skeleton off-guard due to easily blending in. Assassins can be a constant issue too if they happen to catch Skeleton while he's reloading. Big Dog should be fairly standard as a normal run would go, though Snipers are especially a nuisance given the slower speed.

Crystal Caves that is heavy on cobwebs will be tough for Skeleton as he will be moving at a snails pace through them. Laser Crystals attacks can melt through HP very quickly, especially if moving on a cobweb, so Skeleton should play this level safe and not try to rush through.

At this point Skeleton should hopefully be at the higher levels and nearing ultra, he can either choose to continue onto the Frozen City or farm rads from the Hyper Crystal by continually killing Laser Crystals spawned by it. Frozen City gets very dangerous on 5-2 and 5-3 as Dog Guardians, Golden Tanks and Lil Hunter all in the mix is a lot for Skeleton to try to handle while inexperienced with him, so sometimes farming some rads is the safer yet more boring option.

Skeleton should be able to reach level Ultra by the Frozen City, largely dependent on how many rads he's been able to collect. Plutonium Hunger can make a pretty big difference here because of his slower speed and pickups disappearing quicker on loop. Once Ultra is reached the achievement should be unlocked and the run can be ended. Skeleton will have a choice between two Ultras, being Redemption to become Melting once again or Damnation to further Skeleton's build. If wanting to continue the run from there, Skeleton's character guide can be followed.