Rock Band
50 Achievements
1,000
80-100h
Xbox 360
Top Artist
Unlocked an Arena in Solo Tour on Medium, Hard or Expert
10
How to unlock the Top Artist achievement in Rock Band - Definitive Guide
That achievement unlocks once you finish the sixth tier of songs during a solo career. You don't have to do anything special, despite what the description says. Just keep playing songs in a career and you'll get it eventually.
For this one, just finish the sixth tier of songs in any solo career on any difficulty except Easy.