Okay, many people have already told you many issues, but I'll point one no one have yet:
Many guns will target the same enemy at the same time, when a single one would destroy it just fine. Thing is, not only they will target the same enemy, but if it is destroyed, ALL the shots directed at it will vanish in thin air.
This wouldn't be such an issue if there weren't levels when MANY enemies pile up on top of each other. That is - many guns target at the same enemy, but when it's destroyed, the shots vanish when they could very well collide with OTHER enemies on top of the destroyed one, or with another neemy who crosses the line of fire.
On later levels this becomes really annoying really fast. By targetting ONE enemy from the hundreds onscreen at the same time, half of your weaponry becomes USELESS because their shots vanish dealing no damage.
I have played this plenty, dude. One masochistic week straight. I got far, very far. But I can't take this anymore.
You get one star for the graphics and idea. But mechanically this game is a mess.