This has to be done in the new Survivors Game Mode.
This took me around 30 minutes to work out how to do it. But finally got it down.
This CAN be done in a PRIVATE 1 vs 1 Deathmatch.
From experience and about 8/9 attempts, this WONT count in a team deathmatch (at least when you're doing it 1vs1)
So first things first:
1. Setup a Private Deathmatch with 6 slots, 5 private.
2. Invite one friend to the game and whoever is going for the achievement, select Jake.
3. Meet each other in the level, the user who is playing jake, using his Fist "Weapon" - Hold LT and beat the crap out of the other player. Once they've entered a dying state, press RT to finish them off.
Once the game ends, this achievement will pop on the statistics page. Once again, I only managed to get this to unlock in Deathmatch, not Team Match.