Similar to the Where's the Flag, you need to get the blue version of yourself into the next room, but make sure it's in a position where you can cover the spawn point in a green block and still move back to the original room
You need to put 1 green box on the switch, then the other on your spawn point, then move to the next room. The key to this level is making your spawn point as far into the 2nd room as possible, so you need to push both the blue character and the yellow box into the 2nd room
