For sniper try meybe:
Barret Thermal+Silencer
Spas + Grip + anything you like
Tactical Insertion + smoke grenade
Bling Pro
Stopping Power Pro
Ninja Pro

You can exchange barret with intervention. You'll get one-shot kills that way(that is if you hit them in chest), and you'll scare the heartbeat sensor users. Silencer will make you invisible, unless there is UAV. If it is on just go on rampage with spas(it makes one-shot kills too thanks to stopping power).
You can get Steady Aim instead of Ninja for barret-shotgunning, but take g18 instead of spas - Steady Aim actually makes it harder to aim with shotguns(they usually kill at close range without it and you just reduce the spread)