Was doing this recently on my paladin and had been blitzing through the ranks as OS ret to get the bonus invite for a guildie when I hit a wall at dark summoner. The melee range on those ghosts is deceptively long! I found myself thinking that those ghosts wouldn't be one shotting me if I was in my main prot spec... so I done just that - chain defensive cooldowns and tank those ghosts in the face, get all the vengeance and he's super easy! They're somewhat tankable as prot if you had cds running instead of the usual "lol1shot!" and the big vengeance meant I kept up a steady 70k dps, more than enough to complete this whilst trivialising the hard part =)