After killing some very tough boss I usually through a grenade to rend his body. I know he won't stand up like some other zombies, but with this game series you can never be 100% sure about it ;)
I had a bug on lvl 18. The boxes turned into both a box and a ball and didn't move (overlapping animation). Buttons didn't function either. Had to reload the game.
Endless grind, lots of bugs, boring minigames (well, there are some good ones), useless items in the shop, poor animation, no progress comparing to the previous part...
I play it only for badges - not for fun.
sorry, this is a bug