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
I personally used the AK74u for this achievement and it is my favorite Assault Rifle. It has the highest amount of power of all the AR's and actually takes only 2 bullets to the body to kill someone. It has very high kick and slow rate of fire. If you are new to the game I suggest the 552 Commando. It has the best all around stats. Good power, highly accurate, and decently high rate of fire. If you are a more experienced player, I suggest the FAMAS. It has the highest rate of fire of all the AR's. Really high amount of kick, but if you can keep the gun level, you can just spray bullets in a general direction and hit a person.