The base design is good but is overall boring.
The base design is good, but the waves of enemies are repetitive, uncreative, and boring. There are way too many waves of enemies before the bosses, which are themselves not that amazing I'm sorry to say. The weapons are diverse enough in ability, but are never amazing to use. The enemies have so much HP, that killing them just to get through the level is like a chore. Any decently powerful weapons require you to be in direct line of fire, which I understand is supposed to be the trade off, but is fairly suicidal to use near the end of the game. Even before that using them requires hit and run tactics, which makes killing anything take forever. Practically every enemy has the same tactic to deal with, hit and run using the shield as cover, and requires lots of it, since enemies have way too much HP. Near the end of the game enemies cover so much of the screen in bullets that dodging them isn't even an option, and the only thing you can do is use a weak weapon to hit them outside of their line of fire, which takes FOREVER, since as I said before, enemies have way too much HP. I know you tried to mix it up by adding enemies that can attack 360 degrees but while this increased difficulty and required you to take them out 1st, is didn't make the experience feel any less like a chore.
Overall the 2 main problems with this game is none of the weapons you use, while diverse, are never all that amazing to use or look at, and the enemies have so much HP and lack diversity, that killing them feels like a chore. The bosses are just not that amazing and killing them just FEELS like you pelted them for a while with your weak weapon until they gave up, rather then giving you the feeling that you actually DESTROYED them, like pretty much all the enemies in this game.