Stage 1 : Simple, much less trap than BG1. Platforming is mostly filler in this game anyways.
Boss 1(Orn Emperor) : Alright, maybe people who saw him first time might feel disgusting, but I personally feel it's hilarious. Anyways, incredibly easy boss after you know all of its patterns. Kind of a trap in the end.
Stage 2 : Pretty much same as Stage 1. Fillers.
Boss 2(Baldis) : Pretty mediocre boss. There's a charge pattern which can be pretty hard to react, but everything else is easy. Kind of a trap in the end.
Stage 3 : Harder fillers. Jump refresher part can be hard to newcomers in I wanna series.
Boss 3(Rakhris) : Cool boss. The shield mechanic and the patterns he got is pretty unique. He also has a counter attack, so be careful when he stays still.
Stage 4 : You might notice the ambient of this stage. Other than that, just another filler platforming.
Boss 4(Guardian General Rot) : That boss in the BG1! With new cool effects. He got some new cool patterns too. Great boss.
Stage 5 : Just one screen with emptiness. lol
Boss 5(Ecaep dlrow) : He flips your screen, but it's not that terrifying since he has no super-precise pattern. Mediocre boss.
Final stage : You might notice the ambient of this stage too. Also these traps.
Final boss(Gustav Counterfeit) : Cool boss. It has a lot of unique patterns. Also i don't know if it's because of his sound effects, I feel like this boss has a lot of coolness. Anyways, it's a pretty hard boss.
So overall a great boss rush game in my opinion! I won't spoil anything about crazy mode, so that you can play it if you're interested.
This game has a tiny storyline where you kill some bosses to give final boss his power back and then you kill him too becoming the Best Guy.
However there is Crazy Mode which is harder version of normal mode w/o platforming and you play as final boss, there is also EXTRA FINAL BOSS on this version.
Probably worth playing.