Level 3-7 (Mario vs. Donkey Kong: Tipping Stars)
From the Super Mario Wiki
The Minis start in different areas, but the Key Mini Mario is trapped behind a wall of Rock in an alcove at the top. Throughout the course, a few other Rock walls are jamming the space between platforms, and numerous Purple Conveyor Rivets form connections between these platforms. The free Mini Mario has to climb up to the other toy, using Purple Conveyors and picking up Hammers to clear the Rock blocks on its way. Once rescued, the Key Mini Mario has to drop along with the other toy to the bottom of the stage, where the Goal Door is positioned.
The M Coin is behind a wall of Rock blocks, in an alcove on the left. A Mini Mario has to pick up a pair of Hammers and crush these blocks to access and collect the coin.
Nintendo 3DS version
Wii U version