The level Top That! is in the... 80's? section (it's on the right, level is the right-most of those levels - Thank you Pedle Zelnip) of the Foxington world. There is nothing exceptionally tricky about the world or hidden enemies, but make sure to get the enemies along the way of the sinking platform, which are optional as you progress. For the sinking platform part, you can always backtrack and raise the platform again.
If you are defeated during the process, you'll loose a life and go back to the checkpoint, but the defeated enemies will no longer be in the level, and still count towards progress. This has been tested and confirmed (don't judge me).