Wait, what! You're max level and still on that? This should be a walk in the park for you!
But anyway, you should be defensive. Since you're aware that more enemies spawn, make sure you dont go rushing ahead, sacrificing your companions for a "Quick-Kill". Also, picking the right companions would help, such as El Toro, and Bonnie anne. Maybe using Valor's shield or Tritons song if you trained it would help a bit. Try not to discard all your powers, since it is quite a lengthy battle, and you'll need the most you can get. And chain lightning could possibly kill all the enemies, if you're lucky enough, so try it. You never know if you'll get lucky enough. Try saving your bonnie's heal until you're very low or dead, since if you do it at high hp you'll gain a tremendously small amount, and if you do it and somehow die the following turn you're dead for good, and have no way to heal out of it.
And if you kill the boss, i don't think they'll keep spawning.

My best strategy was to put Valor's Armor on my Necromancer pig and boosting his will and damage and turning him loose. He could take out an enemy by himself in one turn or have them so wobbly that any of my other companions with an additional hit after they take someone out would easily finish them off and start in on someone else. If I got my Scratchy in the same battle I would protect him too and those two witchdoctors would clean house. Bonnie Anne and El Toro were my best second mates, Anne for her burst fire and overwatch high damage and El toro would boost the low accuracy of Scratchy and then Toro would just be, well Toro and slice and dice his way through most anyone.

Thanks to everyone who replied your suggestions really helped and I got past it earlier today