This achievement is very similar to the "Gold Master" requires you to get 2 diamond level challenges complete. These are the highest possible to gain. The idea is to get 2 challenges in a single run of the song, so check the challenges required and go for one that suits your abilities best. The chances are that for most challenges expert is required, so "Bleed It Out" by Linkin Park is a good choice.
Note: The achievement will not unlock if you have previously earned a medal on that song, you have to gain all 3 for the first time.