Yikes, this game is pretty hard on normal. Maybe I'm just bad though. :^)
The enemies have invulnerable frames on wakeup so you have to back away everytime you knock them down. I found the 2nd boss to be the hardest so far (I have only made it halfway through stage 4 though) Its just a couple of electro goons.
Also every stage except the first seem to have a gimmick. The 2nd stage has operation rooms that you can destroy to make the boss easier (the more you destroy the less goons you have to fight). Stage 3 has the same gimmick as the last game, the whole "move too much get stunned" thing. Stage 4 as a roulette that locks down one of you moves, the moves change every couple seconds.
The gimmicks arent that bad, but it might be frustrating for some.
All in all,