You will get this while working towards Deathmatch Champion. Once you've won 25 Deathmatches with the max players set to 2, you'll get it.
The fastest way to get the Deathmatch wins and kills is to have two people, set the max players to 2, set the kill count to 1, the level to Hoodlums Backyard, and just knife each other in the back. You can also use the console command "samRestartMap ()" without quotes to restart the map instantly, rather than having to wait for the post-game timer to countdown.