Proving Yourself: Silver Damage

how the fudge do you do this with a dk? O.o

Ok, I have a problem here. I tried this one over and over again and every time I fail at wave 6. Passed once but then failed at wave 7. What am I doing wrong or what is missing in my specs:

Lvl 100 Destro Warlock, iLvl625, Observer as minion. All specs:

http://eu.battle.net/wow/en/character/argent-dawn/Marsultor/simple

I use Cataclysm to kill the rabbits, during the whole wave I keep Rain of Fire up on the healer and try to hit him as much as possible but each time the clock is my enemy. I can't kill the healer within the time of the wave.

Can anyone help me to get my specs right and give me that extra power to get this done?

I'm a demo lock and I've had problems in there also. I can get lvl 8, but healer keeps healing all. What do I do to get this so I can do heroics and mythics? This very frustrating I lost one of my main spells when blizz nerfed us again in Draenor when they took our fel flame from us. Now, they're talking about taking our morph. They already hurt the locks so much with all they've taken so far, why take more? If I still had the fel flame, I'd have had this many times over.