Your toon cannot kill Desther. By reducing him down below 900 HP (about 20% damage), he will surrender automatically for the plot. I've found it much easier combat by leaving my hench helper near the door entry to the area fully buffed (if possible) standing ground, and force Desther to leave the ritual area where you command the hench to attack in the hallway.
Your guide is right about killing the ritual golems which are relatively easy to dispatch. Desther won't leave the chamber until they are all eliminated.
Having several assorted potions to use for buffing, and especially full heals in a no-rest area, should make your goal much easier to reach.