
I use Lancina/BraveIke/DancerInigo as my core for arena and regularly seesaw between 19 and 20. As long as I get a no death streak, going from 19 to 20 is guaranteed. No merges, but full SI for everyone. I usually make the bonus unit red if possible for coverage - using Arvis right now.
I love running into armor teams because they give me more points and my units are tanky enough to take hits on enemy phase. All mage teams are my bane, especially on maps with obstacles where I can't just walk up to them and smack them to death. I rarely run into full cav teams anymore, but Lyn and Reinhardt are splashed into a lot of mixed teams.
As for the recent proliferation of armors on defense, I figure now that people can make full armor teams they're starting to realize how much easier it is to get defenses with them. I've been running one for a while, and as an example, for the first two days of this gauntlet I got over 10 defenses. So I decided I'd switch to my flier team for a change, and since then I've gotten no new defenses. Feels bad man.