The primary challenge in this achievement is getting a group of 8 people together. You're going to create a player match, but before you choose Crusade choose a different game type such as deathmatch or whatever, it doesn't matter. What matters is being able to choose a specific map to play on. The specific map to play on is Necropolis. After you choose Necropolis go back and choose Crusade as the game type. If done correctly then all 8 people choose the Human side and capture the enemy position. The game will be won after only one round as opposed to having to play 3 on all 5 maps to win the game. This method is guaranteed to save you some time.
EDIT: Not all game types have Necropolis available and I'm sorry I don't remember specifically which ones do. But after you do find it and then when you change the game type back to Crusade you have to highlight game type then press A rather than cycle left or right to choose it. This is the key to making the map stay on Necropolis rather than switching to the default map for the game mode.
EDIT: When you choose the map & game mode you have to press the A button to choose. Do Not Press Left or Right To Pick Your Option! Here's exactly what to do: go to deathmatch or team deathmatch and press A on the map then choose necropolis and press A. Go up to game mode, press A, choose Crusade, press A. The map will change to High Pass (not cool). It's ok, just press A again on game mode and choose crusade again & press A and then the map should have changed back to necropolis. It took me a while to get this right.