camp your a** off but don't stay i one spot to long. i haven't got it yet but that is what tip i got from someone who has got it.
i don't have this done now or anything but i do agree with everyone when they say camp your ass off. to camp is definitely not my style at all, i hate sitting in a spot etc but when i was getting my DSR 50 sniper gold i had to camp a bit to get the bloodthirsties. BUT, i never stayed in one spot. get 2-3 kills in a spot and then move to another location. also key would be shock charges and/or claymore or betty and scavenger.
No I would not do it in hardcore. Why would you? Think about it. First off you do kill them any quicker with a knife in hardcore then you do in core since the knife deals 135 points of damage. Second you can't use the map as well to figure out where the other team may be. Lastly asn most important, you will get shot 9 times out of 10 if nothing more then a stray bullet. If your getting shot at you can run and hide and heal up where in hardcore you don't have that option. You get hit you die. If I were you I'd do what I did to get the diamond knife. Run in core capture the flag, TDM or Kill Confirmed. You may be the best team player in capture but it will likely be the easiest.
As far as the class you use goes what I would use it no primary or secondary. Ghost and Lightweight, You can either run with no second perk or fast hands. I ran with no second perk myself. Then the third perk it's kinda open for you. I run Extreme Conditioning and Engineer. You can also run Dead Silence or Dexterity to be able to climb quicker. Or even tac mask to avoid stuns and flash bangs. It's up to you but for me I liked to run Extreme Conditioning and Engineer. Then the scorestreaks I would run UAV only or UAV/Counter UAV but the truth is your going to be running so much with this class that Counter just helps your team mates more then you. Sounds selfish but it get the job done.