If you can complete it, the Master difficulty "The Classics" challenges will award you enough experience to go from Mastery level 0 to 12, as each of the 3 difficulties has a set of "The Classics" challenges and they stack, whilst also netting you the following:
I used the following video by the fantastic MrFreeze2244:
And here were my end of level challenges/exp screens with this being my first run through of the level:
Following this you will easily be able to clean up the remaining 3 levels whilst completing the 5 Mission Stories on any difficulty of your choice.