Minus some bugs, I'm now done with the second full dungeon. (More than half a year later than I should have finished it, but that's better than abandoning it.) It includes three bosses, like the first dungeon did. I think that's probably going to be pretty typical.
The second and third bosses, as well as several of the normal battles, involve enemies that summon more enemies to the battle. Since you only have one party member in this dungeon, there's no reactive targetting for healing or buffs, so the summoning and the invulnerability buffs help add some reactive targetting for offense instead. I'm really happy I was able to get the summoning to look nice in the Chrono Trigger style battle system. I have some that appear in a flash of teleporty light, and others that run in from off screen.
It still starts lagging sometimes when summoned enemies die, and I cannot figure out WHY, and it's driving me insane. It's not even normal lag; the framerate doesn't drop, the CPU and memory usage don't go up, but the ATB slows down slightly and most of the player's keystrokes are ignored as if they didn't happen. Doesn't happen consistently, though! Spent almost twelve hours on this problem already and still no solution. It'll probably come to me when I'm trying to sleep or something.