Well, initially at least it went poorly - at the start of the next fight a Chimaera waltzed up to my MC and one-shotted him. One reboot later (not that I need the lost money at this point), I took a more cautious approach to the fight and won with only two units unable to fight any longer. I used up all my healer's MP right at the start fixing the damage from the Chimaerae, along with most of my mages' MP. I think I missed at least ten physical attacks between all the chimaerae flying in at the start, only seemed to be able to hit them with magic.
After taking them out I crept forward cautiously. The other enemies on the stage were all a bit weaker and I was able to take them out with physical attacks, leaving just the Colossus to deal with. Which was pretty easy - bait them out one at a time, then move in with the whole force and try to take them out before they got an attack in. That worked for the first two. The third had bolt 2 (or was it 3?), giving it huge range and the ability to hit a whole lot of people at once. I sent one of my MP-deprived mages in as bait, then rushed. Unfortunately, I'd had to hang back pretty far to avoid the spell, so the few units who could reach the enemy didn't manage to finish it off. The MC missed his attack, which would have done the job. So I copped a shot of bolt, but fortunately it wasn't enough to kill the MC (Amon was less fortunate) and I took it out before its next attack.
I'm now at what looks like the final battle (or maybe second-to-last). Plot-wise, I clearly need to rush to take the boss out before things escalate, but game-wise first let me just egress, raise my dead, and save.