I can't picture this happening if the DRK OT was properly using only DA Souleaters and Deliriums. He had to have been using Power Slashes, which not only would pull threat off you most likely, it's technically lower dps than had he been doing Deliriums. Now if this is sub-50 content, and he had no access to Delirium, then it'd make sense, as Power Slash and DA Souleater would then be his main methods of attacking.