Although the other solution has mostly correct information, some is left out.
As said, you MUST host to get this achievement, the developers were not good about stating which achievements had to be hosted -- or this is a bug and you're not supposed to be required to host.
You must host a Team Deathmatch, Devastation, and Capture the Flag match.
What's missing is that not only must you WIN, you must have the highest score of all the players.
Team Deathmatch Scores : Kills
Devastation Scores : Kills, Bomb Detonations, Bomb Disarms
Capture the Flag Scores : Kills, Flag Caps, Flag Saves (running over a flag dropped by a killed enemy)