Halo Wars

As soon as you finish your first Xbox Live Matchmaking match this achievement is unlocked. You do NOT need to win but if you do win the following paragraph is irrelevant.

If there are other members in your team you need to stay in the game until you are eliminated. You are not penalized in any way for leaving after being eliminated.

This is more of an add-on. You don't have to play the full thing, you get it whenever it ends.

I know this because I was playing and the other guy quit so i got the achievement, so i assume you'll get the achievement if you quit yourself, but i don't recommend quitting.