I also tried something else just to see if worked: do the same thing you did with the goblin bosses. If you have a save right before the nightsong fast travel to moonrise.

Use a silence spell on Z'rell and kill her. Stealth kill the eye.

Do the same with the necromancer.

Recruit the gnolls.

Kill everyone in the prison.

Fast travel back to nightsong.

Battle just got much easier.