I can't remember which map, game mode or even weapon loadout got me that killstreak, sorry. But this is probably what it was:
Wasteland
Barrett .50 for one shot kills at range (use Stopping Power) & silencer to keep 'em guessing.
Play defensively and cautiously (protect an area of the map, but move around or you will eventually be killed as more players note your camping spot).
Killstreak rewards get you most of the kills: Predator missile gives you a nice bump up to Harrier which (if there are no Stingers around) gets you to Chopper Gunner quite easily on an open map like Wasteland. Again, if no anti-air is about the Chopper Gunner (which is capable of a much higher killing rate than an AC130) will quickly pile up the bodies.
Play Domination as you get extra clues as to where the enemy is and the game goes on longer, allowing you to make the most of your killstreaks. Nowt more frustrating than getting an AC130 as the game ends.
Oh and a nicely placed Claymore can watch your back. I also use a smoke grenade to mask my withdrawal from an area if the incoming fire gets too heavy!