Annoyingly addictive! Didnt realise just how many hours I'd spent on this game until I just checked the clock. Personal best is currently lv12 single player and lv19 2 player.
Dont know if its intentional or a bug, but during 2 player mode you can ressurect your dead bro by upgrading his life at the end of each level. I ended playing both characters I'd focus on P1 and allow P2 to die early on, and focus on levelling P2s damage and max health after each boss. Once P1 was starting to struggling and was low on health at the end of a boss, I'd cure P2s HP and resurrect him for the next level.
As I hadnt needed to waste his powerups on curing his HP up until this point he was generally pretty powerful 200+ max HP - 8 damage. Now I'd just alternate between characters upgrading one until the second could barely continue then swapping between them.