Healing and how it works.
The Flesh Prison will have a green bar below it's health bar, this displays how close it is the healing and summoning it's Spawn which shoot lasers but have very little hp, there is also a Stronger Spawn that has more health and shoots stronger lasers.
The amount Flesh Prison heals is dependent on how many Spawn are remaining. Flesh Prison's green bar will go down when you damage it, and depending on how many spawn are left, will go down even more when hit. (More spawn = Quicker Healing)

Another quirk about wacky boss.
If it tries to heal but you kill the minions before it does, it will enter the healing animation but not heal, however this will give it a large amount of damage reduction during this so it is inadvisable if you want to maximise damage potential.

How not to die
Spawn Lasers - The spawn will continuously shoot lasers at the player which can deal rapid damage if frequently hit, the one Stronger Spawn will shot explosive versions of these lasers.

Blackhole - Not much to say really, just keep moving and don't double-back on yourself and if you have ULTRAKILL style ranking then it won't inflict hard damage so you might be able to swiftly heal back the HP loss.

The Virtue Beams - If you already have enough momentum to dodge them then just slide-jump to keep your momentum, if you don't either; dash-jump or slam-jump then slam down at the peak of the jump and slide-jump once you hit the ground to gain enough momentum to get out of the beams.

Blue Homing Skulls - This is the most important attack to understand as it can be easily taken advantage of in several ways. The attack has two different stages, one slower firing and one faster. First of all the Flesh Prison, unlike in the other attacks, cannot heal during a stage of the attack and will instead delay it until after attack stage is finished, this means you can deal a lot of damage here if the opportunity is right.

It also has a lot of easily parriable projectiles which can give you health and more importantly stamina which is a rather limited resource in this fight, another way to take advantage of these projectiles is that they can be exploded into the enemy shooting them just like Malicious Faces which makes an accurate parry or Sharpshooter shot deal incredible for damage.

How to Kill :steamthumbsup:

(Simplified to not account for variables more information on those after the "How to kill" section)
1. I start with the Screwdriver than immediately then punch it out for high damage, and then shotgun swap + knuckleblaster until it spawns it's minions.

2. Dash-jump then slide-jump to keep my momentum circling the arena over and over, use coins and sharpshooter while doing this until they're fully on cooldown, (except save one SS for the homing skull attack) then projectile boost/revolver the remaining minions. During this period never stop moving around the arena, and you should rarely stop slide-jumping.

3. Use the sawlauncher with magnets to get massive dps, (however only do this if you have a fair bit of time left on the heal, otherwise some of the saws will linger and damage the prison on the next healing cycle, or you can just use less saws so this doesn't occur.) railcannon if it's off cooldown, then shotgun swap + projectile boosts + knuckleblaster if you're close enough.
Repeat steps 2 and 3 (and improvise a bit with cooldowns n' such) until it is dead.


And here you are, now prepare yourself for the ultimate reward.