If you want to power level the best way to do that is just to spend a lot of time killing. I'm sure of the rates, but it seems the longer you play a match the more EXP and GP you get. So in theory the best match to power level would be a small CTF level where there is a lot of killing going on, but it is understood that capturing the flag is not the main objective by both sides. Basically have fun getting a bunch of your friends together to play a full match and pray that their isn't an asshole that comes in and ruins it for everyone. That one asshole is the problem with power leveling.