Dont know of the "best" way but by missions and other stuff. Im lvl 9 after a few hours of deathmatch. When you go on a kill streak you get x1.5 RP and level up faster.
I have a KD of 0.30 lol.
I cant work out the whether aim is auto aim or not, sometimes it does sometimes it doesnt.
depends on the host and what he sets it to.