I did pretty much the same, fireball and firewall. use some missles as AE on the remaining soldiers if any, then you can easy hide behind a pilar to gain up mana / hp if needed so.
Hint: Keep your pet out so they have another target to focus on so you don't have to take all of the hits, if you can also summon a demon do so, and the fight becomes even easier.
Magic blast the divine / zandalor depends on who you want dead first, I found it easier to kill the divine first cause zandalor heals him self.
You can toy with him to get all your abilities to cooldown by hiding at pilars and such, hes immune to charm he can be stunned though.
By the way I did it, I fireballed just as Viru, then ran in pretty much and firewalled, the soldiers were all down and Zandalaor was attacking my demon, I killed the divine with magic blast and just watched Zandalor fighting my pet waiting for my cooldowns to go, I magic missled him a few times then magic blast him again and he died, end of story