well, I must say that the true reward of KF is not the destination, but the journey. That said, I would also check out the "falseplayers" mutator for headshots. Of course, if you farm headshots eternally, then when your done, then you won't be good with it, but only have a higher perk level.
So I told you the cheap*** strategy for those who want to waste their time, now I will tell you a much more fun strategy than endless congo lines of clots being headshotted with a 9mm.
It is: try mixing some perks together a little, but in intelligent ways that won't hurt your experience. Like try being a medic with a LAR, and get down hipshooting the thing as you pick off stragglers. Thats how I managed Lvl 6 SS: just use the LAR and pistols with other perks when necessary, and you will find it levels insanely fast.
As for commando...well, I always found it sort of flawed, the whole "stalker" thing, which is why I myself never really got too far with it. So, in my book, go ahead with that. Playing 100% legit with commando will level probably the slowest; stalkers are rare, and ones that are reserved to one person, rarer. I myself never found the overwhelming need to level, but if you feel that way.
However, Darkwave's advice is the most important. Try and push yourself in difficulty, you level faster in normal play that way than by farming on low difficulties. And you have
waaay more fun at it.
