Sorry, I was assuming you meant 4v4 cuz BTB isn't very MLG. I mean, don't get me wrong, there is skill in it. But I imagine if you want to improve your game you are talking about your 1v1 battles, which are more frequently found on Rumble Pit/Infinity Slayer.
Hit and Run tactics my friend, hit and run. That is what improves KD more than anything else. Knowing when to rush and when to retreat. Map positioning plays a big factor, of course.
The best advice I can give you is to get good with the BR and DMR. Those are the biggest killers, right there.
Here is a good, albeit overused class.
DMR
Magnum/BoltShot
Promethean Vision
Resupply
Explosives
Also, what control setting do you play on? Try Bumper Jumper if you haven't already. You will find the ability to jump and aim at the same time to be very valuable.