It's good. I do agree hitboxes feel a bit big.
I got 60 seconds. No real complaints or bugs aside from. You can shoot through the wall if you're very close to it. I used that to get rid of the autofire and/or shoot enemies before they can get close sometimes. It Feels like a bug, but I'm not sure it should be changed.
Tactics I used
At first, I tried standing up against a wall and then shooting 9 shots in the exact same direction, so they'd be a predictable and then avoidable threat that still clears the first few shields. I think there's potential in that, but it felt more effective to never bounce bullets deliberately, shoot the first few enemies through the wall before they enter the dodecagon.
I think trying to deliberately hit something by bouncing off the dodecagon is too risky. Instead I'd shoot slightly right/left of the shields and then quickly move in the same direction so they follow and point their shield away from the bullet. Still hard, but likely to clear the bullet if you miss. Also just, they all move at the same speed, so they'll collect into one predictable threat if ignored. If you're making levels, maybe have one with mostly shields / with shields added to other enemies, but in a square/pentagon/hexagon instead of a dodecagon, to emphasise bouncing more.
Also, tried to stick to the middle of the screen in terms of up/down, since it's easier to see enemies coming from the side. And did not play in full screen. That's potentially an improvement you could make! Allow adjusting how zoomed in the level is. Having the level smaller on the screen means it's easier to see every part of it at once, but doing that with browser means you'll accidentally click outside the game sometimes. I do like that it's hard to see enemies coming from certain directions, though.