Once again, KILLS are not the only way to score points/XP in this game.
There are MANY ways to do so. I'll give you an example
Engineer+EMP Grenades = score galore.
I play HC exclusively and when I throw an EMP grenade and hit 4 enemies with it, and they get killed, I get 200 points for free. Destroying equipment is another way to rack up points with EMP grenades. Running Scavenger along with EMP grenades means as long as you stay alive, you can rack up INSANE amounts of points.
Another way to rack up score is to pack a launcher, preferably the blackcell, and shoot EVERYTHING out of the air. UAV's are 75 points, and CUAVs are 100 points.
For fun one night I went into a game of HC domination with NO PRIMARY and packing only my Blackcell, danger close, 2 semtex, 2 EMP grenades, Ghost, Scavenger, and Engineer. I wound up second place with a score of over 5,000 points, 7 caps, 14 defends, got my cerberus out twice, RAPS out once, and 3-4 hellstorms. We won the game 200-106 (or something like that, it wasn't much over 100).
Too many players overlook just how EASY it is to score in this game by trying to only use their gun.