Firstly, this can be achieved on any level and any difficulty setting.
For this method, simply start a new game on the Tadpole
difficulty (easiest) and wait for the cutscenes to give way to gameplay.
Try to defeat the enemies that spawn as quickly as possible whilst avoiding being hit - once or twice is okay but any more than that and you should reload the checkpoint. The reason for this is that you'll need to get an A rank
(or S) on each battle on the level (with the exception of the mandatory boss fights against the large pig) to get a 'secret' collectable. There are 4 fights in total with increasing number and variety of enemies.
Tadpole should make this a relatively simple task - don't worry if you mess up on the first playthrough as checkpoint reloading is quick & easy, and there is a stage select once you've completed a level for the first time.
In between all of the fights, in the background of the level, will be floating yellow prisms. The first you encounter will be right after the initial fight is over although it might not show up right away - there is a train that will appear and disappear frequently. Simply wait for the collectable to appear and snatch it with your chosen toad's tongue:
in the direction of the collectable (the same controls allow you to eat flies that replenish your health).
There will be four more collectables as you progress through the stage - if you periodically stop and use the tongue when there are no enemies around, you should easily vacuum up all 5 collectables on this stage. Keeping controller vibration on will also alert you as to when a collectable is onscreen.
Finally, you don't need to get all of them in the same run - so don't fret if you've missed one, stage select and patience are your friends.