Do what I did and just play a lot of FFA. edit:Why would you want to be good at a terrible game anyway?
1) regardless of whether it's a good halo game or not, I find this game fun enough that I'd like to be better at it. 2) prep for halo 5, which is irrelevant if it ends up sucking, which is possible. 3) closure.
Use dexterity. Don't just go run into battles if your have a BR or DMR or something similar, just stay back and pick the guys off. Stay near Cover, don't go into the middle of the map if you have a BR or DMR just stay around the outskirts.