Doesn't Zandalor have to restart when he defends himself? IIRC there is a countdown timer shown on screen, though not very prominent.
Perhaps you are just at too high a level for the monsters to be of much threat to Zandalor? (assuming his abilities scale with your level)
Have you tried this at the downstairs portals? Three of them should give Zandalor and the other guy more stuff to keep them busy.
Try increasing the difficulty setting, if you are not already playing on hard, and see if that helps.
