Matchmaking Legend G10:
Earn 20 Medals total across a Windows 8 device and your phone.
If you have the Windows 8 version, the medals count from there towards your 20 overall medals. There are some medals on Windows 8 that aren't on WP8, for example completing the tutorial.
So the best way to earn 20 medals is do the easiest medals on Windows 8 and the easiest on Windows Phone.
EDIT: Make sure your 10th and 20th challenge is completed on the phone because if you do 10 and 20 on Windows 8 it won't unlock the respective achievements when you load it up on WP8.