Earthworm Jim HD

Keep playing the game and this will unlock. You can quit to the menu, play single player then multiplayer, all in any order as long as you do not quit the game. Once you have 100 atoms this will unlock.

Also, you can start a level with a good number of them right at the start. After you get them, pause the game, and select restart level. Do this until the achievement unlocks.

littlejay2o02 is spot on with his solution. I would just like to note that this achievement is also obtainable through co-op, given it helps if your co-op partner lets you get as many atoms as you can.