Yeah, the final boss isn't hard on level 70+. I guess you leveled up a bit too much. I'm glad you liked the ending, though.

If you still want Yan strategies, here's what I use to kill them effectively. You need Freya's Dragon Crest ability (kill some Grand Dragons to make it do 9999 damage every time), Steiner's Climhazzard (you need the Excalibur, which you can get after doing a bit of stuff in Treno Auction House) and Dagger's Odin. Having Odin's Sword ability is a good thing too, but not necessary. I'm not sure if Boost increases Odin's chance to instantly kill the enemies, but if it does, you should use that too, although it's not necessary either.

Anyway, you should always save before fighting the Yans. When you encounter 1-3 of them, hopefully you get a turn before they do since they're really fast. Have Dagger cast Odin. If you're lucky, it kills all the Yans. If not, have Steiner use Climhazzard and Freya use Dragon's Crest. They should both do 9999 damage to the remaining Yans, killing them. If they still aren't dead, summon Odin again. They aren't too hard to kill, but I prefer fighting the Grand Dragons or the enemies in Memoria because they aren't ask risky battles as the battles against the Yans.