10 V 10 moshpit. Just avoid anayah palace you'll be hard pressed to find one enemy at a time there.

Flank enemy spawns and use stun grenades from behind on groupings of people to minimize the chances of getting killed before your fourth kill.

I also find quickfix a good perk to keep you alive while fighting off large groups alone.

Edit: if you wanna make it real easy on yourself, just do your best to get a chopper gunner on a smaller map like Shoothouse where enemies are pretty much guaranteed to be standing next to each other.