Stealth:
Go for like a UMP-45: Silencer.
Some may call it nooby but its a great gun and the silencer makes it a great stealth class.
For your secondary go for M93 Raffica with a silencer.
Use your knife when you can, flank and stay away from your team mates as they just draw unneeded attention to you.
Also got for stuns not flashes. I just like them more
Go for the perks:
Marathon or sleight of hand
Stopping power or coldblooded (mega stealth)
Ninja. Or if you dont have it just go for like steady aim or something.