The underwater room is very easy if you exploit the little glitch where you make the enemies spawn without dropping into the room. All you do is hang on the ledge and then climb back up, then backtrack to the area with the truck and the enemies should have spawned in the room, making it A LOT more manageable.
Also, I have gotten the platinum on LBP! Up next is probably Mercs 2, since I still need like 8,000+ kills on Resistance 2 to get that platinum.