Guns to use:
SMG
Msmc: This is a good gun to use because there is not a ton of recoil and really powerful
PDW-57: This gun is good to use because it has beter accuracy than the MSMC(Not by a lot)
Assualt Rifles:
MTAR:I strongly reccomend the MTAR because it has almost no recoil if you put the "FORE GRIP"and if you have FMJ you will be unstopable!
AN-94: At first i didn't like this gun but when i put FMJ and FORE GRIP i absoulutley loved it!
Game Modes!
My favorite game modes to play in are HARDCORE SnD and Hardcore TDM
In HARDCORE SnD and HARDCORE TDM i promise you you will get much less hitmarkers wich will not only level you up but will level your gun up because you will be racking up kills
HARDPOINT:
HARDPOINT is a great game mode to level up in because in HARDPOINT if you capture the hardpoint you will get 200XP and if you get a kill in it i think you get 125 or 150 extra points!
Scorestreaks:
I Use the UAV Hellstorm Missle and Lighning Strike.
I like the UAV because it is easy to get and when you call it in every time your teamate gets a kill you get 10 Extra Points!
I like the Hellstorm missle because it is super easy to get if your in hardcore and very powerful. You can also use this a predator missle but i dont reccomend that.
Finally i like the Lightning Strike because its almost the same amount of points as a Hellstorm missle and SUPER SUPER Powerful!

These are the ways that i level up fst in Black Ops 2!
I hope i helped you:)