There's 12 levels in this game and 9 of the levels have 5 challenges that you have to complete. The first challenge on each level is just to complete the level, after that you must replay the level to get to the next challenge.
Most of the challenges are pretty straight forward tasks such as collect all stars, break all boxes and beat the level in a certain time limit. Then there's a few others that can be tricky such as only using the flippers a certain amount of times, slaloming through stars and scoring points every 5 seconds. There's also two challenges that you must find secret areas and there's one level you have to find a hidden gem (I show where these are in my video)
Most of the challenges aren't that difficult, only a few gave me trouble. Slaloming through the stars took me awhile and the one I thought was the hardest was the second challenge of the level Kuton, which is the boss. You have to kill the boss with only being able to use each flipper 20 times. Overall it's not that bad, just keep trying and you'll eventually get it.
I go over the achievements and the completion in my Achievement Review video and also I show where the hidden gem and the secret areas for the challenges are at 5:30