It is possible, if you are buffed enough to go directly after Varen. If you are a warrior and can beat him down quick enough you can potentially avoid getting too many hits from the attacking Dremora, which if I recall correctly are warlocks who are pretty decently leveled. So, if you can end-around Varen to get him between you and the Dremora, you will be more difficult to hit and he might take some friendly fire, helping you out in the process. Poison on your weapon can help as well.
I think Varen is a Dark Elf so don't bother with Fire enchanted weapons or spells, they won't do enough damage due to their resistance unless you've got a really strong weapon and the skills to use it. A shock enchanted weapon helps to suppress his magic regeneration.
If you are an archer/thief, you'll have to bob and weave between the crystal formations to stay away from the spells the Dremora are throwing at you while you shoot at Varen. A Shock enchanted bow helps because that drops magica levels and both the Dremora and Varen will regenerate health with restoration spells when it drops too low. Poison helps in this case as well.
If you are a mage alternate between shock and flame spells on the Dremora. I don't think they have the same fire resistance that they did in Oblivion even though they are just re-textured dark elves. The higher leveled ones will have Alteration skills though so will carry a certain degree of Magic Resistance. If you hit them with a strong enough frost spell it might slow them down enough so you can run over and start in on Varen.
Otherwise just carry a bunch of potions with you or go back to a save before entering the star and work on leveling and buffing your character up for the battle. As mentioned, the Dremora will be leveled to meet your character, but you might have learned a few more tricks and skills that will give you the advantage by then.