There's Another Way
There's another way to get Mastery rather than Mighty's Dojo. I was playing Pig Lair today and I noticed I got some Mastery for Red when I beat some pigs in battle. After that I went to play some other levels with my weaker birds hoping that I would get some Mastery for them. I didn't. None. But I did realize something. Thinking I got some from Pig Lair and not other battles I think it depends on level. Because the pigs there are level 33 and so am I. So as a simple guess with some proof is that you get Mastery if you knock out pigs that are the same or higher level as you. I will test with the Daily Dungeon tonight. But until then this is just a hunch. I also realized that when you do get Mastery it goes to the bird who knocked it out.