What level is your character, and what build? Have you summoned the creature, and another summon, if available?
If you've killed all the paladins, move to the opposite side of the room as Zandalor, to work on the Divine One first. If Zandalor doesn't have a line of sight, he'll probably focus on your creature, so you'll just have to deal with one at a time.
Hide in Sight can be useful to get a break and/or re-position to attack again. For an archer, Way of the Ranger + close range Splitting Arrows is very effective, otherwise use explosive, poison or stun arrows. For a mage, just keep spamming Magic Missiles, and Fireball or Magic Blast. A warrior should be able to just use Rush Attack, standard attacks and whirlwind if there are multiple opponents.
After this fight, you'll be able to visit the battle tower before the next combat encounter, so don't worry about using up any consumables you have (stat and armour boosting potions, etc), and health / regeneration potions can be used up as long as you can still make it to the end of the fight.
