Do you have other things left to do? The easiest solution would probably be to come back at a higher level, when you have better equipment, and can do damage faster. If not, in a save before starting the final section of rifts, head back to your battle tower and upgrade enchantments (particularly Increased Damage and Increased Magical Damage, and if you are hanging on to any malachite gems now is as good a time as any to use them), use any charms you may have been saving, etc. You could first check available merchants for equipment upgrades, if you had not done so since the last time you levelled.
What class is your creature? A mage head would probably be better than a ranger head, and either better than a warrior head, IMO. A demon summon could also help, if you have a skill point or two to spare, or that bonus on a piece of jewellery.
If you stick close to Zandalor you should have an easier time protecting him. I stayed just in front of him to cover those two rifts, occasionally having to Rush Attack a memory eater from the rift in the back. I used a bow to attack opponents from the front 2 rifts (mostly the right one); it didn't do much damage (hadn't invested in any ranger skills), but it did help draw opponents to me, rather than have them go directly to Zandalor (my creature mostly got the attention of the opponents from the left rift).
I was level 35 or 36 when I did that section, one of the final things before heading to the hall of echoes (duel wielding maxed, level 10 Bleed, etc).
If you are not already playing on easy you could temporarily lower the difficulty.