What is the most difficult boss you defeated recently? It doesn't necessarily have to be the hardest boss you've ever fought, just one that you had some trouble with as of late.
For me, that would be Diciple Aihtirol from Xenoblade Chronicles.
This boss took me quite a few tries before I could defeat it. What it likes to do is to summon these elementals, that not only boost it's defenses to a ridiculous degree, but had some pretty high defense themselves as well. It didn't help the main boss was really big, so it liked to push me and my party members into the ether pools that surround the battle arena.
It took me a while before I realized I could use Melia's AOE magic attacks to quickly get rid of the elementals in order to get rid of the main boss's defenses. And even then, it took a couple of tries before I finally took it down.