i got my diamonds by staying in the middle of the action. i camped in the middle of that snowboard map. right in front of the gondolas.
I've got mine diamond and the way to do it is to use it like a sniper (ie hardscope and QS when necessary). Also play an objective gamemode like DOM where the spawns are fairly stable. Position yourself near the action but not in the action and use shock charges to watch your flanks. Be sure to put the shock charges back far enough to give yourself ample time to turn around and aim before the enemy is on top of you.
I'm on Ballista too, but gave up a week ago due to the amount of sh*tmarkers I get with it.
Don't play HC with a Ballista, and here's why. The Ballista is bolt-action, so you have to cycle the bolt or reload cancel after every shot. Which means if someone were in close range to you in HC, you'll die unless you can quickscope. Also, 9 times out of 10 if you snipe someone on HC, they're coming back to kill you sooner or later (in my case asap). Another thing, if you insist on sitting in one spot, don't. Move around in a general area with a decent view of the enemy entrances to that particular area. The Ballista is effective in core, but most people don't like to use it because they have horrible shot placement. But I recommend Demolition/CTF/Ground War. Demo since you won't have to worry about a spawn flip (unless your whole team deside to prance around in their spawn. This is my top recommendation for Bloodthirsty medals. CTF because of the same as Demo, the spawns won't flip. You can camp your flag with ease on CTF, only downside is the spawn delay. Ground war lastly because of the amount of people, can be a hinderence and an advantage in some cases. Try to play Team Deathmatch on Ground War on a decent sized map or atleast one with decent vantage points, (Plaza, Turbine, Carrier,) etc. Higher chance of death, but also a higher chance of getting Bloods quicker. Or just learn how to quickscope. And I'm not condoning camping btw, but it's your game do what you want. (I've had diamond snipers since the end of January and 14,500 DSR kills, 2700 Ballista but I QS a lot so I don't like it). Good luck, hope I helped.
Also, a good class setup for the Ballista for Bloodthirsties is:
Ballista - Ballistic CPU + Extended Mags
Perk 1 - Flak Jacket
Perk 2 - Toughness
Perk 3 - Dexterity + Tactical Mask
Tacticals - 2 Concussions/Flashbangs/Shock Charges
Wildcard - Perk 3 Greed
This is effective, you don't have to worry about being stunned/flashed, explosives won't kill you in the offchance someone throws something your direction/at you. Dexterity can be swapped for Extreme Conditioning since you won't need to raise your gun faster, but it helps in quick escape i.e mantling through a window. Never use a sniper rifle without toughness imo, the flinch without it is horrible unless you've manage to adapt to not using it. Ballistic CPU since you'll be ADS'ing a majority of the time and Extended Mags so you have 10 shots to get 5 kills incase of misses/hitmarkers. Another tip, the Ballista is OHK fgrom the chest up, so aim for the upper chest area.