Sarasaland Adventure v4.0 (SMBX) - Subterranean Symbiosis (7-3; both exits)
Info: https://rpgmaker.net/games/4216/
In this level, you need to constantly switch between Mario and Peach to progress; some areas require Mario while others require Peach. Although it isn't really difficult (compared to other World 7 levels), there are some spots that are difficult to get through (like the two small rooms where the yellow and green switchblocks are, and the underwater room where the second star is.). You'll also have to work with a few switchblocks and spawning enemies that come out of the pipes in various areas. How far you can get here depends on who you're playing as at the time, and if you find your progress halted, that basically means you need to find a player block and change characters or find and activate a switchblock somewhere.
The first area features a steel pipe which can take you to one of three other pipes; hitting the ? block above it will change the pipe's destination - you'll need to use that knowledge to destroy the two metal barriers that block the exit leading into the vertical area.
Getting to the secret exit basically means you need to get to the end of the level, take those bombs that shoot up out of the pipe there and bust the key free, then carry it all the way back to left side of the room where the keyhole is. Opening the item house here is a good thing because it is the only item house in the game where you can acquire Hammer Suits.