In effect, Death Knight is a relatively complex class to master. How do you fight against this class? It has so many defensive mechanics and can put out a beastly amount of DPS on the battlefield at the same time. We will give you a more thorough aspect of the Unholy DK powers since a majority of Death Knights are unholy. There are a few things you should learn when you encounter DK face to face.
   To cast Bone Shield when you enter arena so the CD is up when the gates fall down. This will allow you to have 8 bones as soon as you enter the battle because you can raise another set of bones when you want to.
But you will try to auto swing and use all weak possible direct damage abilities you can to waste and consume the charge on the DK's bone shield. This can't be purged so if you have casters you can have them try to wand it/ice lance etc. This combined with IBF makes a DK very hard to take down. For mutilate Rogues, don't muta right after you stun, wait for your white hits to consume the bones then use muta. In particular, the muta is wasted when you muta against a charge of bone shield. 40% damage reduction is pretty big.
When heavy fire is coming towards or a Rogue is trying to set up a devastating kidney shot, you can use Icebound Fortitude. When a BM hunter uses Beast Within you should use this ability because he will try to burst you down when his pet stuns you with intimidate. Being able to be immune to the intimidation and cut his super saiyan power damage in half will save you from dying. While the DK is immune to stun, he is not immune to other CC's. Remember that Death Knights are vulnerable to CCs so blind, sheep, freeze, fear etc while you see IBF up.
Magic Suppression will make you immune to magic damage while under the effect of anti magic shield even if the pom pyro was going to crit for 10k, it would initially hit you for 2.5k damage but with this talent maxed out you are immune. You can use AMS instead of combining AMS and IBF to survive super spell damage. There isn't much you can do when a DK is running around with 10 second spell immunity unless you are physical DPS. Other than that, just try to heal up and keep topped off.
Gargoyle is when the DK DPS shoot up the foot, basically a flying moonkin just spamming wrath on the Death Knights' target. A good DK would know how to use his Garg. There are a few tactics for your reference. One is that you can try to train one person down and go all out on it or launch the Garg at the DPS and work on the healer. Another is that you will put the healer in a tough spot because he needs to decide who to heal. The Garg can hit from 1.5 to 3-4k per shot depending on the Death Knights' AP ATM. To make this more potent, summon the Garg out and if you have a Shaman blood lusting it will yield an awesome result. Presences have an effect on the gargoyles damage. With blood out it will shoot for 15% more damage and with unholy its normal damage but shoots 15% faster. But shooting faster drains runic power at a faster rate.
At present, it's bugged and comes out with half health around 5.5k hp. If you are ranged then you should shoot it down ASAP to mitigate a lot of incoming damage. If you are melee then you are screwed and you will have to live with the Garg out shooting at you or your teammate for a minute. Or you can stall the DK long enough so he runs out of runic power.
In a word, we hope you can know how to fight against Death Knight well and enjoy this role in the game.