Probably the easiest way to kill the final boss of the game.
Final Boss Guide
Place of cheese
The final boss room will have 2 ball shaped enemies and then the boss with his hands and head sticking out of the window at the back of the room. The position of the cheese is to stand right in the opening to the right of his left hand (your right). between his thumb and forefinger. If you stand here, both of the ball-shaped enemies in the room will bug out. The red ball will still fire lines of fire but it will miss a little each time and for the spike ball it will just sit there and do nothing.
Now you can take damage from the main boss but this is where your build comes in. You need to stack all of your skill points into as much Life Steal as you can. The easiest way I’ve found is to put Fearless Slash as your primary skill tree and stack any shards you have that will give bloodthirst back immediately (ie. Bad Grip, Cold Blood Burst) as well as absolutely any skill or shard that increases damage (more damage = higher healing) and obviously any skill that increases healing. This is the build I used to fix the main bosses damage. If you use this build and spam build burst every time you charge it and rinse and repeat for each stage it will be the easiest boss ever.