Each level has a total of 15 challenges you must do in those levels. There are a total of 180 challenges in the game. Almost all of them are very easy or take little time to complete. You may replay levels to work on different ones if you need to. After you finish a level, almost all of the stats you have in specific challenges will be reset. For example, if you have 1/5 enemies defeated in a certain mode, and do not accomplish it by the time you finish the level, when replaying it, you will have 0/5. Try to grab all of these on your easy playthrough. Check the guide below for a full walkthrough.