Thanks for the help everyone! I used Riki, Melia, and Sharla, and leveled from 7 under her to 2 above her. That solved the fight

Turn the game difficulty down. Grind another 10 levels or so. The last bosses are way more difficult and this boss fight acts like a gatekeeper.

I've never had any issue with this boss with more ether-focused builds (melia, riki, and ether 7 are great). I didn't even realize it was notorious for being difficult until after i had beat the game lol.

Is your level high enough? XC is not a hard game, usually it's just a matter of having a high enough level, decent gear and a balanced team... or Shulk, Rein and Dunban