I found an easy way for this achievement, but just got around to typing the solution.
1. Get a group of 5 - 10 people together. If less than 8, then someone needs a 2nd controller. You need at least 8 slots filled out of 10 to make this totally painless.
2. Everyone search alone or as a team of 4 in Versus > Forces of Nature > Guardian, until you find a game that is empty on your side, and has 1 enemy on the other side.
3. Read out and message the gamertag of the enemy. Everyone then needs to send the enemy a friend request, and it does not matter if they accept or not. Just send it. From the main menu, press X. Even though friend request is not accepted, it should show them as friends. Half the party should join their game so that you can fill up the enemy side. At the same time, the person that found that match and is on the opposing team of the enemy, invite the other half of your party. This will allow you to fill up both your side, and the enemy side. Works much better than searching.
Then just get the random to work with you, or troll him out of his game.