Use the smallest deathmatch map. Cant remember the name but its 2+ players. Have the killer stand with his back to the crates and facing the nade launcher in the middle of where you are standing. Your mate must stand in the very far back corner of what you can see (Behind the Armour). Then just keep firing the nade launcher at your m8 he will always respawn in that corner. You average around 50-60 kills in 3 mins. Now when firing if you move forward to get ammo before your m8 respawns he will respawn else where. This will also net you double/triple/quad kills with only 2 players. The guy who is dieing muat spam his RT button to respawn stright away.