MoA, C7 boss: what the hell am I missing?

So, I'm fighting >!Gonska!<. It isn't too bad, until after his second break. Then he just gets pissy and completely wipes my entire front row. At first, I figured, "okay, a single big attack to corner you a bit. No biggie." Then he wiped out my back row. Up to this point in the game, I've obviously had some fights where I've needed to actually think, manage buffs/debuffs, etc, but nothing close to this.

The recommended level is 31, my team is 34 at least, and while I wouldn't say I'm using the best gear, it sure isn't the worst. Elem def is 200 minimum up to 400.

To be clear, I'm not new to Octopath at all, but I'm obviously doing something stupid here that I didn't have a problem with during 1 and 2. The things that are jumping out at me are:

• a painful lack of multi-hit elemental attacks that he's weak to, simply to break faster.
• something to handle these massive row-wide attacks that one-hit my crew

So, any pointers? What am I fucking up, here?