When I originally entered the College, I never bothered completing any of the quests, influding the original "Tour of the Campus" quest. I just kept coming back for the training I needed. So Savos Aren, Mirabelle, and Ancano have been standing around the central area for weeks now.
When I summoned the Unbound Dremora and began my battle, another Dremora Lord suddenly showed up and joined the battle. At first I thought the Unbound Dremora had summoned him, so I attacked him, and he was pretty pissed about it. But I tossed him, and then killed the Unbound.
On the second summoning, I watched more carefully. The Unbound Dremora was not summoning, yet a Frost Atronach showed up and beat the hell out of him. I noticed the Frost Atronach had come out of the Door to the Hall of Attainment. So I looked over the side, and there's Savor Aren, summoning creatures and sending them through the front door of the hall.
After I finished the battle I went back to a saved game and tried again. On Legendary, it turns out that all I had to do was summon my Storm Atronach, summon the Unbound Dremora, and step back. Arvos would send in his Dremora Lord, and the two would kick the Unbound Dremora's ass in about 30 seconds. On the second summoning, Arvos sends in a Frost Atronach, and again they put the Unbound down in 30 seconds.
So - having trouble? Just wait until there are Professors in the Courtyard below before beginning your battle with the Unbound Dremora, and let THEM clean up for you.