You have to play smart when only using a knife. You can't just run around in the open. I would suggest moving around or even hiding (depends on your playstyle) in close combat areas such as the two main buildings on highjacked. Also, If you are running at a sniper, make sure to do the retard run, as it actually works. (the retard run is where you look straight up and run at the person while quickly looking left and right) Another important aspect is your speed. Make sure you have lightweight on so that you can run quicker at enemies.
Knife classes are a lot of fun, but the perks you pick make all the difference. First of all, I woudl recommend using 5 perks and not 6. While it's nice having 6 perks. If you can make do with 5 it will give you a full compliment of grenades to work with. First, pick out your perks.
Light weight is essentical here. Flak jacket is nice, and ghost can help in certain game modes, but you can likely make do with only one perk here.
Tier 2: This depends on the rest of your load out. If you are running heavy on grenades, Fast hands combined with scavenger can make a knife class nasty. Otherwise, cold blooded is a good alternative. Or just pick one of the three.
Tier 3: Not much questions here extreme conditioning and tactical mask. Maximize mobilty, no stuns grenades.
Your equipment depends on the perks you are running. If you have scavenger concussion grenades are the best tactical choice stunning an enemy. 2 of them are great if you are running 5 perks.
In your lethal slot, c4 or the combat axe are the best options. Also, don't be afraid to bring a launcher along.
Do exactly what Darkrangeresp says mate, that's good advice. I just got diamond specials, what a grind. But, don't run straight at them, you will die. Use the zig zag run and confusion of flash. I found when going for the 5 bloodthirsty medals the best game mode for me was domination. Cap a and b or c and b then get in their half and hide while watching the map. Wait till you see someone run past and pounce. Wrinse and repeat. I even found waiting in doorways or round corners was good as the enemy sprints past just knife em lol. By the way, don't forget if you are using blade only, the trigger will work instead of shitty thumb stick. Hope this helps? ;o)
Even though I've got weapons mastery and usually only play HC I get bored from time to time and go on a knife only or pistol/tac knife rampage in ground war, it's a good way to have a bit of fun.
My combat knife class is usually
It's geared more towards survival as those pesky dickscopers that inhabit ground war will always try and C4/flash you if the retard run defeats them. Engineer is good for knowing where to head for free kills on tac inserts as well.