There are a couple different ways of obtaining the needed:
- You can save the Gems up from the Mine, Daily Advanced Quest rewards, and various other places or purchase the gems with real money from the Shop under the Bank tab. Then, you can use the gems to purchase the maximum Mastery Scrolls needed for a champion.
- The other option is to go to the Minotaur's Labyrinth and get the Mastery scrolls as a reward for defeating the Minotaur.
The tactic that I use for getting the Mastery scrolls would be to get four strong champions and use gems on those champion's to unlock all of the Mastery scrolls. Once you have those four champion's you can have the fifth champion that you want Mastery scrolls for in the Minotaur Labyrinth lineup, and by doing so you will filter all of the rewards to that one champion. Freeing up your gems to be used in other places.