Easiest way is to play a multiplayer two headed giant match with at at least one other friend. Make your friend on the other team and get him to leave and you should get the win. Easy, easy! This should also unlock 2 other achievement, one for winning an online match and the other for winning a 3 player match, 4 player match, or two headed giant match. Goodluck!
Important comment from EarthboundX: -Alright, we figured it out, you have to uncheck the AI takeover box. Then when you quit, the game will end.
Seems simple now, but this was my first time playing the game.