Arm WrestlerG 40

This is the fourth costume you will unlock.

This is unlocked in the fourth level, 'Cosmic Punters', in the third world, 'Blue Cosmos'. It's just to the right after you start the level. You jump over a pit, and then you'll come to a horizontal fork of pathways. Take the lowest pathway and you'll find the costume key. Pick that up, and the achievement will pop.