Wait in the Xbox dashboard (outside the game) have a friend start the final mission on No Mercy. You need to be in contact via microphone.

When your friend calls the first radio he then needs to invite you to the game.

when you reach the No Mercy loading screen (the one that tells you what player is what) you tell your friend and he calls in the second radio.

Now when you arrive, just complete the mission.

It doesn't matter if you or anyone else gets hit, it will still unlock the achievement.