Using fast firing weapons (machine guns, mechs, etc.) at big targets is really helpful. You'll probably want some teammates to take care of smaller targets while you unload as much ammo as possible into big ones (like bug tanks). I did a mission where you have to kill a number of enemies and at the end I had ~1500 bullets fired with 81% accuracy. If you have other teammates helping and knowing what you're doing it should be easier, though.
