You must achieve 5 stars on each of the songs in the Hi-Def set on Medium. The achievement will stack so if you 5 star all of the songs on Medium you will unlock Easy as well.
The Songs [And Difficulty] Are:
**Run (I'm A Natural Disaster) 
Mai Ai Hee 
This Is How We Do It 
I Wish For You 
You're a Jerk 
Sexy Chick 
Somebody To Love 
** Unlocked after 16 stars.
It is worth noting that some of the songs are VERY difficult, particularly in this set list, so don't overwork yourself!