Limitless

Fuse + to kill everything on screen, even while the the boss is invulnerable.
Fuse a second with to get extra stacks to trigger faster plus extra damage from curse.
(you start with the balls for this from your characters) fused with gives you insane constant healing as long as you have enemies on screen.
If you got the perfect balls so far you'll still have 2 slots left so you can get a third and fuse it with for extra stacks to trigger even faster, with your last slot you can get for an insignificant amount of stacks, for a bit of extra healing or for a bit of extra damage, none of these will really make a difference.

Comp explained:
hits all enemies on screen every time it bounces, deals 30% of enemy's max health as damage every 3 bounces, this means you'll kill any enemy in 12 bounces no matter how much health they have, I was able to reach depth 2000m in Endless Mode with this then I quit cause it felt like there was nothing left to do.
heals you every time you hit a charmed enemy, thanks to the fusion, all enemies are charmed at all times so you heal every bounce.
is the only mandatory passive, aim for the back of the screen to get lots of bounces and trigger lots of Hemorrhages.
If you have any ideas on how to improve this build please let me know!