In World of Warcraft Player versus Player mode, Death Knights can be the most difficult classes to fight. Depending on the spec, they handle different threats in many unique ways and have an answer for everything that comes their way. Far from invincible, yet extremely powerful, if you can over come certain pitfalls then you will be well on your way to finishing off Death Knights much more often.
First keep in mind that a Death Knight doesn't need to have you nearby in order to afflict damage upon you. Depending on your class, they are just as comfortable fighting you from far as they are from close range. But from experience in playing, you will be able to tell what the DK wants from you. Hence, good proactive you will help lead you to battle victory.
For example, if you are a healer, he is going to want to fight you up close. These are the squishier level of classes, therefore keep in mind that he will want to keep you in melee range most of the time. And if this is the case, make sure not to keep close distance, otherwise, your change of survival will be much more difficult.
Your goal will be to keep your distance from him, because he will try and get you as close to him as possible. Then increase your distance and fight from there! Fears, roots, blink, are will help you get away so make sure to have them handy. He will likely use his trinket because of your crowd control abilities, which is ideal for you since it will open you up to control the fight with more of an advantage. Bad for you, this will motivate his to death grip you into melee range and of course you must escape again and finally that will give you the overall advantage.
Melee classes fight the Death Knight up close and at near range which of course will prompt the DK to kite them. Do not fear, just have your snares up. Including hamstring, crippling poison and frost shock just to name a few. Implement this tactic and he will most certainly get up close and personal. You will find him snaring with his Chains of Ice making your strategy that much more important.
Do more damage by directly attacking the back of the Death Knight. Since majority of mitigation is countered this way, you will be able to inflict much more damage and begin the road to victory.
Don't forget the World of Warcraft Death Knight is not afraid to stand toe to toe with a different melee class. The plates they wear protect them from damage and they will not have to only resort to kite you. Just be ready for the kite if your tactics do not work for some reason. You never know and may blow an offensive cool downs to burst him.
Bottom line is that the DK is easier to defeat than it has been in the past. The best tip is to understand what they will be trying to do to you and this alone will give you the best upper hand in the fight. Of course, there are more things to be careful of depending on your class, but our advice here is going to be a great starting point for you!