Won Gold on all Playlist 7 Challenges in Co-op Mode.
Playlist 7 has a total of 4 challenges:

(1) Siberia Survival

Gold Medal Requirement: 05:00:00 min (solo) - 05:00:00 min (co-op)
Preferable Mods: Double Health - Double Armor - (Explosive Ammo (as a gunman) OR Top Speed (as a driver))

(2) Moscow Air Support

Gold Medal Requirement: 30,000 PTS (solo) - 30,000 PTS (co-op)
Preferable Mods: Double Health - Explosive Ammo - Any

(3) Hong Kong Drift Battle

Gold Medal Requirement: 70,000 PTS (solo) - 70,000 PTS (co-op)
Preferable Car/s: Orchid XGT
Preferable Mods: Double Health - Double Acceleration - Double Boost

(4) Moscow Rampage

Gold Medal Requirement: 8000 PTS (solo) - 8000 PTS (co-op)
Preferable Mods: Double Health - Double Armor - Magnetism

Tip: Use your magnetism mod to make cars stick to your ball, then go to the exploding barrels around the map, it will net you around 500-600 PTS for each car you explode.