SpongeBob SquarePants Underpants Slam!

In order to get this achievement you must play quest mode.

The first time you play a level there will be 5 bronze medals randomly scattered throughout the levels. There is only one medal present at any given time and they do NOT spawn in the same locations every time.

The second time you play a level you'll be going for 5 silver medals (I believe it's only 3 silver on the first level for some reason). These silver medals seemed to spawn randomly on the blue jellyfish. I highly recommend that you kill any jellyfish you see while going for your silver because I found that sometimes the level becomes flooded with other colors of jellyfish and then there aren't any silver medals to be found. While not required I always recommend having your special saved up for when you come across one of these silver medal holding jellyfish. This is actually the most difficult of the 3. Also, always try to have your special ready so you can kill the jellyfish when you see them as they can sometimes be a real pain to kill.

The third time you play a level you're looking for 1 gold medal. These gold medals are found in special objects in the level such as oysters, lockers, and a weird looking water cooler type thing. These will NOT have been previously on the level in your previous play throughs of the level. So they are pretty easy to spot. They require your special move to break.

The best way of going about this to use Spongebob for the Bronze, Patrick for the Silver, and Mr. Crabs for the Gold. That way you kill two birds with one stone as you can get your medals and progress towards your story achievements.

Also, just a general tip when going for the silver and gold, make sure you always grab any dark blue cross bubbles you see as thy fill up your special bar.

The other thing I did was generally on each level there is a breakable item or two with underpants in it off in a corner somewhere. Go about freely collecting the underpants but don't break your "saved" breakables until you've collected all the medals first.

It does NOT count if you get all the medals but not all the underpants. So if you can't find all the medals but you can find all the underpants go ahead and collect all the underpants as you have to at some point replay the level with Sally and Plankton at some point, but do be sure to make note of it as you're going to want to play this game as little as possible.

Play quest mode. The first time you play the level you need to collect all 5 bronze medals. Play the level again and you will have to collect 3 silver medals which are carried by blue jellyfish. Now play the level a third time and find the 1 gold medal. The gold medal is found in an object that requires your special move to break. These objects are lockers and oysters and such. Do this on all nine levels and you are done. Pretty easy achievement, plus play through all bronze levels with one character, all silver levels with a different character, and then a third character for the gold medals and you will have 3 of the 5 characters done with quest mode towards the 'Masterful' achievement for 30 more gamerscore.

Thanks to ZeroDesolation for a few add-ons to original post.