With each of the two portals upstairs you need to protect Zandalor for 15 seconds in order for him to close the rifts; downstairs with the three portals at once you need to protect him (or not) for 60 seconds. I think the countdown timer shows how much time is left until Zandalor can finish his spell (didn't pay much attention to it).
I was level 36 when I did this quest, and Zandalor restarted the spell a couple times while got around to summoning my creature and getting the hang of the pattern of spawns. At least downstairs you shouldn't have a problem letting Zandalor get interrupted at level 28.
