How do you get diamond combat knife in black ops 2?

It depends if you are talkig about the ballistic knives or just a regular knife. Ballistic knives are unlocked at a certain level as a secondary weapon. A regular knife (to knife someone) is automatically equipped. Just press down on the right thumbstick and your character will knife. Be sure you are close enough to knife, too. The way Ballistic knives and the regular knives differ is that you can thow a Ballistic knife. A good way to practice knifing people is to play Sticks and Stones (wager match on Live), and to learn the basic skills of knifing, I highly recommend playing campaign mode, as they show you how to do a lot of stuff that you can also do in Live, which I found very helpful.