To get this achievement, you have to feed a Gordo Slime food until it bursts.
Gordo Slimes are unusually large slimes that are found in set locations.
Remember, Gordo Slimes are not Largo Slimes! Largo Slimes are large slimes that form when a Slime eats a Plort that does not belong to their kind.
Gordo Slimes are very large slimes that always exist (until you burst them) and are unable to move.
The easiest way to obtain this achievement is bursting the first Gordo in the game. To burst him, you need to feed him 30 units of food. (Thanks to Figbender and SwedelSlug224 for confirming)
Follow this video to get to the first Pink Gordo
http://xboxclips.com/Avenge+Agony/1ee1d0f2-4bb4-450a-9b1c-d6...