Actually i think you are geared enough to do HOR HC, however you have to treat it like an old instance where you just cant lolfacerollaoepewpewmoaraoe.
Unless you are grouped with 3 rogues and a druid, you should have some meaningful CC.
Ret paladins can use repentace and turn evil/undead, Hunters can trap, priests can shackle.
After the cc is taken care of, you need to set a killorder, if cc is available, i like to do mercs->priest->mages->footmen->riflemen, with the riflemen cc'ed.
if no cc is available riflemen move up after priests. The reason for this is the 50% increased magic damage they put on people. It is absolutely lethal in conjunction with a split mage.
I like tanking in the center. it gives a better view of the instance and more room to relocate the party in case of a flamestrike.
Mercs are evil. Burn them down fast.
Mages wont split if you stun them around the 55% mark and burn them after that point - this is ofc hard when you are a DK, but maybe a friendly paladin or rogue could help you there.
Priests arent that bad if they are interrupted when casting.
Riflemen are evil and should be cc'ed.
Footmen are lollerskates and can be saved for last. When they enrage, not so much.
I am the spearhead. I am relentless. I am the last line of defense. I am a Warrior.