Although I am still working toward this I have a nice chunk toward it. The best way I have found to find fast groups of enemies to shoot at is finding one of their spawners/barracks. Once found, hide and they will start spawning is groups of 1-4+. Best part is if you are already aiming in the general area most dont move for like a second or so, so if you're quick you can get a few shots off at them. And most are disoriented when they first spawn.
Other tips are just playing as sniper and camping outside capture zones.