hrm. let’s see, i’m still fairly noobtastic, but I’m almost top100 somehow. I don’t know how, but here’s what I do:
think of the dumbest weapon possible to use in a situation.
buy it
keep getting pwnd until you know where they normally go
if there’s a flank, take it 
(knife flanks will make you a server god in no time, i am evidence of such)
if you keep getting uber-pwnd, save up for the ONLY gun under the machine gun category. buy it and charge the incoming mass of rape. pull the trigger and don’t let go. you will get at least a good chance of bullet-spamming them to death.
if all else fails, buy ONLY a HE grenade, pull pin, run into enemy. suicidal, i know, but you get lucky every now and then.
and the most important one of all:
rush like there’s no tomorrow, and DON’T STOP FOR ANYTHING
enjoy your instant win sauce