BFS Nemesis boosing, watch a video on youtube so you know how to do it. Must have a headset, play the medic class, and be able to revive.
We stand in a circle, on a hardcore server, and shoot each other in the head, one person from each team shoots the other team, the guy alive revives his partner, and repeat. The nemesis bonus gets so high!! You never die because you get revived, so your KDR is 100%, it's the best way to boost.
Add me xxMORG-ZIPPAxx PSN