The description for this achievement can be a little misleading. You must play co-op in one of the Toy Box Game Disks that are included in the starter packages. Place one of them on the hexagon slot on your Infinity base and you're ready to go (NOTE: For those with the digital download version no extra placement is necessary, they are already included in the game).
The Marvel starter pack comes with Escape from the Kyln and Assault on Asgard.
The Disney starter pack comes with Stitch's Tropical Rescue and Brave Forest Siege