This achievement has become glitched after the last patch. It will take over 10,000 kills to net this one.
All the kills MUST be on ranked matches and kills on Annex DO NOT COUNT towards the "10,000"
Personally it took me 12,827 kills to get this one and I've heard of people with 15,000 - 18,000 that still dont have it, so just hope you have some luck and keep at it.
My personal way of getting the most kills is to host 10-12 round matches on smaller maps (Raven Down, Fuel Depot) with a 2-3 min duration (playing with friends helps too, to trade off hosting duties). I averaged 20-30 kills per match.
Just have fun playing and don't give up, you will get it eventually and oh boy will you enjoy it :D