Battletoads

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.

You can get this very easily on the first spaceship shooter level you play (Spacebrawls - Act III Stage 1), but obviously this requires you to have made it a decent way through the game.

Throughout the level, you can easily spot the 5 yellow collectables that drift in from top of the screen and all you need to do is fly your ship over them before they disappear from the bottom of the screen.

The 6th collectible is for completing the level in under 8 minutes. This is easily done if you can just stay alive, collect the power ups & shoot everything without dying (which should be easy enough if you’re decent at space shooters / twin stick shooters) and you’ll finish around the 3 minute mark (meaning technically you should be able to die once or twice and still pick this up with ease).

All in all, it’s much easier than going for this on the main beat em up levels…. And it goes without saying to play this level on ‘tadpole’ difficulty to make your life even easier!

An even easier method I found personally is the very first racing level. I believe it’s like the third or fourth level. Very early in. You have to beat the level in under 8 minutes and 30 seconds. Really not a hard level at all, just make sure to collect the other collectibles in the level and try not to die. I got hit once or twice and still made it in time. Also I found that reloading checkpoints doesn’t work, so it has to be done in one go. This way you don’t have to worry about fight ranks and you don’t have to worry about playing like 3/4 of the game to get to the space shooter level. I got it in two attempts. First attempt I loaded a checkpoint once or twice and that got me over the 8:30 time limit. Second one was all the way through. Just don’t miss a collectible and you’re good.