UPDATE: Exact score requirements added, confirmed by developer.
For this secret achievement, "Platinom Nom Nom", you have to get a platinum medal on every level in Free Play. That means every variation of every game, 42 medals in all (there are six levels of River Rush, nine levels of each of the four other games). Your scores in Adventure Mode don't count, meaning that even if you get a platinum on everything in Adventure Mode, none of them carry over to this mode. So in Adventure Mode, you only need to do well enough to meet the requirements. No need to put in any more effort than that.
Getting this achievement is a pain in the ass. Literally, if you're not a teenager anymore. I don't think I've physically worked harder for any other achievement.
If you have the pre-order DLC, you do NOT need to get a platinum on (or even play) the DLC levels to get this achievement. I know this because I didn't even bother to play the Champion level on Reflex Ridge before getting it.
WolfVelocity's explanation of the achievement requirements was helpful, so I knew I had to get platinum medals on every level in the first place. Now that I have, I can expand on his solution by giving the required scores for each level.
I have personally contacted Good Science Studio, the developer for this game, and have received the EXACT scores required to get platinum medals in all levels. All required scores are multiples of five, with most multiples of ten.
# Indicates a pre-order DLC level.
Leaks Ahoy!: 140
Go with the Flow: 160
Ship Shapes: 185
Sea Saw: 210
Crab Crazy: 215
Ocean Side: 180
Shark Shimmy: 250
Starboard Step: 310
Fish Frenzy: 280
Curvy Creek: 160
Bouncing Brook: 130
Cozy Caverns: 160
Fantastic Falls: 125
Misty Mountain: 130
Rapid River: 120
Sure Shot: 170
Treasure Chest: 180
Trap Doors: 190
Break Out: 180
Secret Doors: 150
Blast Off: 225
Halo Hopper: 230
Solar Streaks: 235
Cosmic Curves: 400
Moon Madness: 270
Lunar Lines: 330
Meteor Storm: 310
Galactic Arc: 330
Big Bang: 365
Hope this helps some of you guys. Here are some basic strategies for getting these scores:
This game is the easiest by far. Make sure your play space doesn't have any obstructions, or you might have some difficulty getting to some of the leaks on the floor in the corners.
When multiple leaks arise, there are two basic types - ones that you have to plug all at once to advance, and ones that you can plug quickly, one at a time. Assume it's the former unless you know otherwise. If it's the latter, be prepared to move quickly, as new leaks will spring up before you've plugged the old ones if you take too long. Just go fast and be prepared to pick up a leg to plug some of the leaks. In addition to your arms, legs, and head, you can also use your torso to plug leaks. The instructions don't mention that.
When playing with two players, try not to reach over into the other player's side unless the other player is having trouble, your side is leak free, and you are sure you can reach that leak. If you and the other player have a skill disparity, it will be much more difficult to get a platinum medal, as there are several points which require multiple leaks to be plugged on both sides before you can continue on. If one of you is struggling, it will hold you both up for several seconds and cost precious time.
This game is also fairly easy, except for the Misty Mountain level, which required several playthroughs for me. The general strategy is to always take a high path over a low path if you have a choice. If the raft is in mid-air, you can jump it if you haven't already jumped it in the air yet. This tends to make up for the lag. Still, don't automatically jump at the end of every ramp or platform. You might accidentally jump over some Adventure pins that way.
For Misty Mountain, stay in the clouds. The platinum requirement is very hard to get if you don't take the cloud path for the entire level. If you fall off, which you will more than once, it might be faster just to quit and start over.
Getting platinum medals in River Rush with two players is much more difficult. Play by yourself if possible. If you must play with another player, coordinate your side to side movements precisely. Online, this may be near impossible. Also, if you both jump at the top of ramps and when going across gaps, this often makes the raft go too high, causing you to miss Adventure pins and platforms. You can remedy this somewhat by having only one person in charge of jumping. If that person misses a jump, the other person can then jump.
This game seems to have a lot more luck than the other games, and as such it was the hardest one for me to get platinum medals on. In general, you want to hit the balls as hard as you can. If you don't, they will go slowly and destroy only one block before returning. If you hit hard, the ball will go much faster, have a fiery halo, and destroy multiple blocks. You need to do this as often as possible when there's only a single ball in play. Don't forget to use your legs. If you have multiple balls, worry more about keeping them all in play than hitting them hard. Your rapid flails will hit at least some of them fairly hard.
If you have trouble with getting hard hits on the ball, it might be better in some cases to let it go by you and then use a serve to get a hard hit much more easily. You'll lose a little time, but it may be much less time than if you hit the ball too softly several times in a row.
You don't get a time penalty for waiting to serve at the beginning of a round, so don't serve until you have a good idea of the level and how it moves. Then, try to get an opportunity to strike a target with your serve. The extra balls are absolutely required to finish the later levels quickly and get an all-important time bonus. Peek-A-Boo was probably the most difficult level for me, as its platinum requirement means you not only need to use this strategy, but be perfect with it for three rounds in a row.
If you play with two players, play with one person slightly in front of the other. That way, both of you can cover the entire side-to-side area of the court without bumping into each other too much. In general, though, getting high scores is all about hitting hard and it's difficult to do that without hitting the second player. It's easier alone.
For the most part, the platinum thresholds in Reflex Ridge are fairly low, with one or two exceptions, so don't feel too bad if you make a few mistakes. Still, you'll want to jump in place to speed up the platform whenever you can, especially on the vertical traveling portions, as a high time bonus at the end is needed for nearly every course.
The pull bars were particularly problematic for me. I can only get them to work 50-75% of the time, and you need them to not only go faster but score a few Adventure pins for each successful pull. Because of this, those courses with a lot of them, like Speedster, were harder for me. Just remember with the pull bars that you have to line up your arms with the handles very precisely, and that you need to pull a little earlier than you think you do to compensate for the Kinect's lag. Pull right when the green lights turn on, if you can.
Getting platinum medals with two players is just like getting them with one player, since you both get the same course and separate pin totals. If you're playing with someone better than you are, you may get credit for a platinum even if you can't do it yourself (though it only counts for you if the second player is local, not online, as all platinum medals must be earned on YOUR Xbox).
Try to be on top of/next to the bubble shooters as they light up. You can get a lot of bubbles with very little effort that way. Other than that, just move around quickly and try to find "sweet spots" where you can absorb all the bubbles in a pattern without having to fly around too much.
In general, you can float up a lot faster than you can float down, but you have to flap your arms slowly and deliberately. If you flap too quickly it won't be detected, so if you need to fly up fast, don't be too frantic about it.
When trying to float down, don't slam your outstretched arms at your sides too quickly, or the game will interpret it as a flapping motion and make you go even higher. Just gently put your arms at your sides.
When playing with two players, you're stuck on your respective sides but you can reach a little bit into the other player's zone if they're having trouble. This is probably the only game of the bunch that is actually easier with two players, as for the bigger patterns both players usually get a shot at taking out all the bubbles.