This is a solution that I am writing for each of the Kill 100 enemies with a specific weapon class achievements. I am currently working on these achievement myself, so the following solutions are what has worked best for me. My current rank is a Major, mostly earned through player matches, so I have been playing for some time now. Alright here are the weapons classes, followed by a detailed summary of the specific class.
Pistols: Glock 18c or Desert Eagle
Sub-Machine Guns: P90 or Type 05
Shotguns: XM26-LSS or Tactical 500
Assualt Rifles: FAMAS or 552 Commando
Light Machine Gun: MG36 or MK46
Sniper Rifle: PSG1 or SR-25 SD
This is the hardest to get out of all the weapon class achievements. I suggest playing on Villa. Use the PSG1 or SR-25. These two rifles use clips and are semi-auto rather than bolt action. They are both one shot kill. I suggest crouching in a corner or hugging a wall and popping out and shooting someone. You can try dropping the armor and running around and shooting people as you come up on them.