You will need to boost this achievement, as no one plays this game online. Luckily, this can be done in Player matches with only two people.
Set up a Player match, game type Team Elimination, on the map Headquarters, with a 1 kill limit and 1 life limit. The map is very small and circular, so have both players run to the open area in the center of the map. Have one player kill the other and the match will be over. Rinse and repeat 50 times. Takes about 30-45 seconds per match, including loading times. You can even start the next match with a simple press of the A button. At the end of the first match, you will unlock the following achievement:
I recommend doing your grenade kills while doing this because there are no grenade pickups on the map and you only have the two grenades you start with. Be sure to stand well back when tossing your grenade because they have a HUGE explosion radius. At the end of the 50 matches you will also unlock: