1. Keep moving. After getting a kill, always change position.
2. Claymores! They're essential in FFA. Well placed claymores can easily make up a third of all kills.
3. Keep cool. Don't go for a revenge kill. Other people are attracted to the radar blip and gunshots, but you'll likely end up bumping into a few enemies you weren't expecting.
Flash Grenade Scavenger Pro Stopping Power Pro/Cold Blood Pro Ninja Pro
Key: Hold down one area and always watch your back after you get a kill, the should be 2-4 spawns to look out for depending on which spot you decide to control. Claymores can help but you are exposed when you are planting the Claymore so be careful.
Assault Rifle of Choice (preferably TAR-21 since it does awesome damage w/o stopping power), attachment: Grenade Launcher Perk 1: One Man Army Pro Perk 2: Danger Close Pro Perk 3: Commando Pro (by far the best 3rd perk with the OP knife in this game)
Tube twice, get to safe location, use OMA pro and switch to your class again for more tube ammo. Rinse and repeat.
Assault Rifle of Choice (preferably TAR-21 since it does awesome damage w/o stopping power), attachment: Grenade Launcher Perk 1: One Man Army Pro Perk 2: Danger Close Pro Perk 3: Commando Pro (by far the best 3rd perk with the OP knife in this game)
Thats what i use when i feel like ruining peoples days. I like a Silenced UMP or Tar: NINJA PRO IS A MUST HAVE Cold Blood/ Stopping Power and Scavanger for the 1st perk.