I just killed the final boss in ONE TURN using Doorbuster combined with Cursed Skull and Symbiotic Clone for a grand total of 120 damage. And although that is a lot of damage, it's not even the most broken build! That would be Flash Sale.
Flash Sale is an attack that deals 5 damage and pierces, which is already pretty good - a lot of enemies in the game will die instantly to this, except for some in the Crypt and all of the enemies in Gehenna. However, Flash Sale consumes ALL of your ammo, no matter how much you have. But you can remove this problem with 1 item: Gun Oil. It'll automatically reload for 5 ammo at the end of the turn if you have no ammo, doesn't use up a turn, doesn't lower your guard ring, and counts as a reload. Infinite ammo for Flash Sale. Even better: Since it counts as a reload it triggers the effects of Bubble and Totem. Therefore, you can get healed for 3 HP AND get a free hit EVERY SINGLE TURN! Since you don't need to worry about ammo, you can ALSO get Mirror, which lets you reflect bullets. Normally Mirror removes how you get bullets from guarding, but since you don't have to worry about ammo, it's free reflection! Plus, if you add Contract, enemy HP is reduced to the point where all but 1 enemy and the boss will instantly die to the attack! This build may not kill the boss in 1 hit, but it's definitely enough to keep you from dying in any section of the game.
I'll be trying to beat Normal Mode without taking a hit now.