Not too difficult, just get a buddy who is familiar with the game and beat it on co-op over XBL. If you get somebody good enough, you can also get team work (will pop in-game once you reach 100k) and full assault (once you beat the game, if under 12 minutes and you are alive) fairly easily. Also, you MUST be alive at the end of the game to get the achievement. If you run out of lives and die and your partner beats the game, you won't get the achievement but your partner will.
EDIT: Ok, so it has been confirmed that you can be dead at the end of the game, as long as the other player survives, and the achievement will unlock, but only once the other player (who survived) has quit out of the session. The player who died must also have made it to the final level in order to unlock the achievement.