This achievement pops when you free 300 of the 700 possible Teensies. These can be acquired by:
- Finishing 'normal' levels, for 10 per level (8 regular Teensies + King and Queen). These are mostly hidden, but should easily be found when scanning the edges of the map, or looking behind corridors. The regular teensies are in the level itself, the King and Queen are behind hidden doors. You'll have to complete a mini-game to free them.
- Finishing 'special levels' (Invasion, freeing a Princess, ...). These have 3 Teensies, who are usually not hidden but often a bit harder to reach.
- Lucky tickets sometimes grant a Teensie.
This achiement popped for me at about 40% into the game. Makes sense, as 300/700 is about 40% as well ;-)