We must be missing some exploit there.
Heh no. It doesn't go up at all (or at least so slowly, it's impercetable) unless you kill CZ targets. Fdev forgot to add experience points for killing anyone. Elite combat rank goes up regardless of what you blow up in your ship. All targets count towards it.
At any rate, this is where I'm at with merc rank:
I've killed hundreds of scavengers and criminals in the last week and they didn't move it at all. That all came from Czs and I've done about 11-12 of those. That's a lot of killing to just about push into the second rank.
Compare that to Elite... It takes about 8 kills to get there. Maybe less. And no one can claim that combat ranking is fast, can they?
Merc rank is currently both broken and way too slow. It's not important, because it's mostly fluff... But it'd be nice if it was fixed. I want to see how the missions scale up as I gain rank and that's not going to happen at this rate.