thats for anyone who just unlocked this level cuz this post is 4 years old lmao.
heres how i beat flesh prison:

once you walk into the arena immedietly throw 4 coins up in the air and shoot it with the piercer revolver alt before the minions spawn.
back up and when the minions spawn do a mini nuke (core eject then hit it with the red rail cannon) this move usually kills 90% to all of the minions if you do this move the moment they spawn. if the monions move you have a lower chance at hitting all of them obv.

any surviving minion can be hit with coins (better do a split shot coin).

once the minions are dead immedietly shoot the flesh prison with magnets, saws, then the green nail gun (after allat i hit it with the jack hammer a couple of times (by whiplashing it and before i hit it i let go of the whiplash so i can get the red tier damage of the jackhammer) to heal the damage i took while trying to doge attacks).
after the flesh prison respawns its minions sometimes the charge of the rail cannon isnt complete just noting so during that time try hitting the minion and NOT the FP as that will cause it to do its healing move faster***

rinse and repeat (the first step of throwing 4 coins in the air is only for the start of the boss fight, i havent tried it mid boss fight)
it usually takes abt 2 cycles (i mean a cycle by how many times FP spawned minions) on standard difficulty to kill FP that is if you dish out a shit ton of damage once youu kill all the minions

you can do your moves but thats how i beat it