I liked the game, especially graphics and arenas, also music is really cool. Bonuses before the start are a good idea. However, can't say, that speed is useful. Shotgun is just bad. I would suggest speeding up player's bullets, because now they are too slow and it's not satisfying.
It's cool, that you added a lot of different arenas, but it would be cool to fix barrel and spike wheel spawning for each level. For example, on the circle arena they just fall in the abyss. AI is often stuck in platforms and can't do anything.
Anyway, cool idea, can be a lot of fun. The game needs some gameplay and UI improvements )