Won Gold on all Playlist 6 Challenges in Co-op Mode.
Playlist 6 has a total of 4 challenges:

(1) Siberia Drift Battle

Gold Medal Requirement: 50,000 PTS (solo) - 50,000 PTS (co-op)
Preferable Car/s: XNN 233
Preferable Mods: Double Acceleration - Drift Boost - Any

(2) Siberia Speed Check

Gold Medal Requirement: 470.00 MPH (solo) - 950.00 MPH (co-op)
Preferable Car/s: Dodge Viper SRT8
Preferable Mods: Double Acceleration - Top Speed - Super Boost

(3) Siberia Rampage

Gold Medal Requirement: 6000 PTS (solo) - 6000 PTS (co-op)
Preferable Mods: Double Health - Double Armor - Any

(4) Germany Self Destruct

Gold Medal Requirement: 600,000 PTS (solo) - 800,000 PTS (co-op)
Preferable Car/s: Dodge Viper SRT8
Preferable Mods: Double Health - Top Speed - Double Acceleration