Guncraft Blocked and Loaded

First, host a capture the flag match.
Set max players to two and private slots to 1.
Next, select the map "Thorsin's Ar".
Change the settings of the match to have a score limit of 100 or however many captures you need, and uncheck the timed match box.
Keep running to and from the flag and earn yourself an easy cheevo!

I used a custom map in a private match by myself to unlock this.

I made a flat land map and added the required items to save and use the map online.

I needed to place 12 spawn points, capture the flag post and capture spot and the monster spawn one.

I made the flag base and capture point next to each other and saved the map. The game will tell you if you are missing anything else in red before it can be used online.

Went to xbox live, made a private match and loaded the map from the custom menu. Adjust the score to 100 wins. You can change movement speed but the bases are so close it doesn't really matter.

I got the achievement in about 7 minutes in one game.