Any tips on how to get more Mastery than "flexible murderer?"

[Garamoth]

Wait, those microscopic xp gains actually count for level mastery?

@schatenjager

Yes, you can gain mastery by simply replaying the level over and over again but the challenges really provide the most rewards. I got to the point in Chongqing where I was at mastery level 18 but there were no challenges left to do.

How is that even possible? Which challenges were bugged?