Ok, achievement is for winning 100 deathmatch games. pritty simple. set the match up with these settings.
Map = Sun place
Max players = 2
Private Session = On
Min players = 2
Kill limit = 1
Time limit = 10
Auto map cycle = Off
Infinite ammo = doesnt matter
Health and armor pickups = doesnt matter
Allow join in progress = YES
With these settings you can get a win about every 45 seconds. The map is small and you can learn it easy. Just trade off wins with a partner and this can be done in a couple of hours for both players.
Hope it helps