I'll tell you what i did:
Unholy spec, Unholy Blight, Lichborne, Death's Advance, Death Siphon, Blood Tap, Remorseless Winter.
One by one, Unholy blight helps with keeping dots on all the elementals that spawn. Lichborne helps keep you alive. Death's advance is really crucial with avoiding the prince's ground cleave attacks and with kiting the elementals. Death Siphon helps with healing you while kiting and at the same time doing damage to the adds. And last the Remorseless Winter talent will give you more breathing windows/DPS opportunities on the prince, for 6 seconds you can go behind him and bring down the hammer.
The images are the easiest part as a deathknight, because of Blood Boil, Death and Decay, Anti-Magic Shell.
I have to say this: on my deathknight, the blessing challenge was the easiest of all of my 5 toons i did this quest on. It's as if every spell and ability deathknights have was specifically tailored for this quest alone. It's ridiculous, and it was a walk in the park. I did the DPS challenge on my deathknight, mage, rogue and hunter, and if i were to rate the difficulty of the quest based on class it from hardest to easiest, it would look something like this:
1.Rogue, 1 try, 1 kill.
2.Mage, 1 try, 1 kill
3.Hunter, 1 shot
4.Deathknight, 1 shot.
The legendary quest doesn't need any nerfs, in fact i would have wanted a harder quest line, and by that i don't mean more RNG droprates from raids.
Edit: i forgot to mention the ease with which you can manipulate the fight as a deathknight by using strangulate and death grip. You shave off precious seconds when the adds are all dead by gripping the prince and bringing him to where you want him to be, or if the adds are up, and you are in a tight spot with the prince's cleave attacks, you can interrupt him again, or grip him away from the adds, not to mention that he will be unable to get a single cast off because of strangulate/death grip interrupting.