Here is a playlist with all the Challenge Dojos completed with Gold Medals:
https://www.youtube.com/playlist?list=PLnM236QQZlrNtAUxNS23i...Amount of Studs needed for a Gold Medal:
The Collapsing Condo: 100,000 Studs
The Slithering Catacomb: 170,000 Studs
The Stormy Docks: 170,000 Studs
The Secret Factory: 180,000 Studs
The Deadly Arena: 220,000 Studs
The Dark Swamp: 90,000 Studs
The Scorched Tomb: 90,000 Studs
The Savage Skyship: 80,000 Studs
Before starting any Dojo complete 'Location 8 - Ninjago City North' to unlock the Master of Endurance Ancient Scroll (Invincibility). Replay the stage again in freeplay and collect the Master of Fortune Ancient Scroll (2x Stud Multiplier):
Completion of all 8 Challenge Dojos also unlocks the ‘Master of the Dragon’ Ancient Scroll.