Ok, this is an amazing game! The animations are well done and fluid. The sound design is pretty good. As Akzidenz said, the rounds are pretty brutal and it feels like the game makes good use of the time constraint. I liked that when you block one attack you receive chip damage. It adds a little to the urgency.

The IA is kinda strange though. In some rounds it just avoids all your attacks and kill you mercilessly; in others you can keep spamming Hook and the IA just keep getting hit. I did it in 3 consecutive rounds :P

Brilliant and very highly accomplished. It looks great, sounds good and plays very well. It fits very nicely into the 20 second constraint because the rounds are naturally brutal and short without feeling like they're being cut down to fit the time limit.

Maybe adding music for the start and end of each round and something playing in the background while you navigate menus would give it a touch more of a GB vibe? Not that it really feels like it's lacking.

Aside from that, the game is pretty near perfect. Great work.

Cool graphics!

Cool graphic and aniamtions!
I mean I really feel impact - this is cool, I like it!

This looks great! have you tried using GB Studio to make an actual game boy game? It is surprisingly accessible, although you may be disappointed about its graphical capacity.  I used it to make this: Microidvania by juzek (itch.io)

Wow, love both the pixel art and gameplay! Great out-of-the-box idea for gameplay!