Once you load in and spawn, you want to take a left to face the rock and two pathways. You then want to take the right pathway, but instead of following it, you want to reach the border of the map. Once you do, you want to continue running along the border until you find a closed off area that looks exactly like this:
In this area, the invading forces cannot reach you over the little hill, and must go around to shoot at you. But, very little will reach your destination due to friendly fire being on. This means that you don't actually have to fire a single shot, as the enemy will shoot at the men approaching your destination, giving you free kills. Wait there until the number hits 100, and you should have every achievement in the game!