This achievement is unlocked by completing the The Savage Skyship dojo. It is located to the left of 7 Pt. 1 - The unclimbable Mountain on the map.
Completing the dojo and getting the gold medal for the Master of Challenges achievement is much easier after you have finished the story, which will unlock all the Ninjanuity Tokens. The skills granted by the tokens will make it easier to keep your hit combo high enough to meet the gold medal score.