You have to solo career on medium, there's 8 tiers total with 4 songs in each + encore, also you only have to complete 3 songs in each tier, not all the 4.
But you still have to complete the encore or boss battle in the tier.
If there's some songs you can't manage to do, practice.
A tip to use in the boss battles is to save 3 Attacks until the end of the song when the opponent has the solo.
There's 3 Boss battles:
Tom Morello (2nd Tier)
Slash (5th Tier)
Lou (8th Tier)
Good luck! :)