I got three achievements (Not Defeated Yet, Field Medic and Team Sponge) at one go using the below method.
1. Load the last level of the first world (rain * something)
2. Join the game using three other controllers (guitar hero controllers work too)
3. Let the 3 controllers idle and let the boss down them. Stand in the green circle where the downed player is and stay in there until the meter is full.
4. Repeat step 3 until "Field Medic" is unlocked
5. Proceed to defeat the boss and "Team Sponge" will be unlocked after the cut-scene.
* Idea courtesy of CRAZE KILER.