So, allow me to brag: I went 18-0 on Karachi
(before the glitches hit). I now have all the accolades! I then went on to do really well on all the maps with the following setup:
SCAR+Noob Toob
One Man Army
Danger Close
Commando
Fire off two grenades, then switch to different class:
SCAR+Noob Toob
Marathon
Stopping Power
Commando
~OR~
AUG+Holographic
Sleight of Hand
Stopping Power
Ninja
~OR~
Snipe+Thermal
Sleight of Hand
Cold Blooded
Ninja
So, basically after the first successful encounter (i.e. your two noob toob grenades hit), you treat that as a decision point. What class do I need next? That's depends on the context. At Karachi I would switch to my Marathon/Noob Tube class. Run around and knife. Or I would switch to my AUG class and perch up high and shoot from a distance.
It's a totally different way to play than what I'm used to. It's much more flowing. At one point you're running, then grenading, then stepping back and engaging in mid-range combat. Very fun stuff. One could have One Man Army in every class if they wanted to but I'm in love with Sleight of Hand so one change during each life is enough for me.