It would be better just to get an AC-130 in the multiplayer, since there are a lot of different factors. You get used to the lag, map layout, finding out where the spawn points are, and how to take out people efficiently.
Killstreak setup for quick AC-130:
Predator Missile
Harrier Strike
AC-130
As for a good class to get it easily... it's all preference. Although, if you like to camp a little...
ACR w/ fmj or silencer
AA-12
Scavenger Pro
Hardline Pro (or Cold-Blooded Pro)
[insert perk here] <--- which ever perk you feel comfortable with, whether it's Commando or Ninja or Steady Aim, etc (But I do NOT recommend Scrambler)