I just beat the Eater of Souls on my Reaper. All I did was swap out 2 of my 3 utility skills. I melee-ed the whole fight. When he sucks you toward him, just let it happen, but when he does the pounce on you to gain health again, use a big CC like Greatsword 5, Shroud 5, or the elite Reaper Shout. Between all that, just DPS hard. I am not sure, but I think it seemed to suck health less often when I was in Shroud form. Maybe he didn't use that skill at all whenever I was in shroud, but it might have been coincidence. I did notice that having a friend with me in this instance did not help much.. if anything it puts you at a disadvantage, because the boss can suck his health too if your tactic is based on trying to keep distance. Any friend you bring becomes a ball of light (Spirit Guardian) with only 3 skills: Might, Regeneration, and Vulnerability. They can only empower you, not actually fight alongside you.