play hardcore team death match for quickest XP imo!

or softcore tdm if you dont like HC mode

I read someting about boosting...GTFO.

I think im like rank...15 star Colonel or something idk, I just play for fun and couldn't care less what people think. That is what is most important, play for FUN!

quickest way I see level 45's and higher level without unlocking items is standing in a rush server, the attacker gets the first 4 mccoms exploded, then the attacker and defender stand next to an mccom and arm/disarm it for like 1-2 hours
every once in a while the defender will kill the attacker 5 times in a row while arming to get the defender award and nemesis award, then on the sixth one the attacker kills the defender while disarming so he gets the nemesis pay back award
and mccom attacker award, within 1-2 hours I have seen players end the game with close to 100k points, they do that a few times thru out the day and get a full rank a day, keep in mind this is for rank 45+ which is tons of points to level

aim gun at the other guy and shoot

worked for me

Make sure you shoot before he does.

Like mentioned before though, play the game to have fun. If you try leveling up every class and using certain guns just to get certain stuff it will get old quick.