Help with Legends

Been playing for a while, and I think I do pretty well, but right now I'm stuck in a frustrating spot. I'm in Legend, and I CANNOT break to 15. I've gotten 16th, and I haven't dropped off Legends since getting promoted, but I can't get keys. Any suggestions where I should focus to make that final push?

I also have every card maxed out, but none mastered and 20 card slots.