I didn't find the last quest that hard. Hardest fight, sure, but I just taught my battle party heal, and used that.
This is basically all I had to do after the first turn:
Attack, attack, heal, heal.
Heal, heal, attack, attack.
Repeat as needed.
And of course I had to be weary of the cooldown on the heal spell.
Won first try without losing anyone. First turn I just hit F and auto-attacked.
I had 'maxed' slave's, having upgraded them in the lab, and the best armor/weapons.