Put on spy plane and blackbird. These are two REALLY good killstreaks that can get your KD up in no time. People think the RCXD is better but its really not. With the RCXD you usually get one kill (maybe two) and sometimes above that if your lucky. With the spy plane and blackbird however you know the enemies location for a good period of time and can get you way more kills then the RCXD can. Plus the recon plane and blackbird help your team out because it lets EVERYONE see where the enemies are so that can lead to more wins which will also raise your win/loss ratio. But for your 3rd killstreak, this one is really up to you. Choose a kill streak that can get you a lot of kills, or one you're really comfortable with. I use dogs or chopper gun depending on what map. If its a map with a lot of buildings and indoor places to hide, use dogs. But if its an open area such as Jungle use a chopper. I would select dogs though because dogs will be getting kills and you can still run around and get kills on your own as opposed to the chopper gunner where your in it the whole time.

Now for the classes. This really depends on the map. I'd setup a couple of different classes. An assault rifle class, sniper, and an smg. I wouldn't recommend using a shotgun because they're shortrange only guns, so if someone is shooting you from long range your pretty much stuck and are most likely going to die. Here are my class setups for core.

AUG Extended Mags (Or Famas if you haven't unlocked AUG yet)
M1911
Concussion (This throws off their accuracy as wells a making them slow)
Frag Grenade (You can throw them farther than semtex)
Perk 1: Ghost (To hide you from spy planes, Pro Version comes in handy)
Perk 2: Sleight of Hand (Quick reload for moments you need it)
Perk 3: Hacker (Save yourself from those nearly impossible to see claymores)

AK74 Rapid Fire (For an SMG its got good range)
M1911
Concussion (This throws off there accuracy as wells a making them slow)
Frag Grenade (You can throw them farther than semtex)

Perk 1: LightWeight: (Comes in Handy when running in space to aim faster and move quicker *help by the campers*)
Perk 2: Sleight of Hand (SMG's have smaller clips so you'll need to reload more)
Perk 3: Hacker (Save yourself from those nearly impossible to see claymores)

1. What I do when I play is I be cautious when I move. Every time you turn a corner aim down your sight so you'll be ready to shoot any near by enemies.

2.USE THE SPYPLANE AND BLACKBIRD! I can't tell you how much it annoys me when theres a spyplane/blackbird in the air and my team isn't using it or is just waiting for someone to come to them. This is bad guys use these killstreaks to actually get a lot of kills.

3. Pick a game mode you like. I don't recommend team death match simply because everyone always spawns behind me and I get killed. Domination is a good mode to play because you know where they will spawn depending on what targets they've captured.

4. Don't stay around in the same spot for too long. (TDM ESPECIALLY) What I've noticed is when you stay in the same spot/general area for too long on tdm (and on some other game modes) people start spawning behind you or near you. Even if people aren't spawning near you, the other team will start going on the hunt for you after you've killed them a couple times in the same spot.

5. Stay around cover. I'm not recommending to camp but just don't leave yourself too open. Especially if you know theres a group of enemies headed your way get behind some sort of cover then start shooting at them. However don't try to out gun all of them. I'd say after you use about one clip on a group of people, get back and find another spot to wait for them. I've noticed that when I start spraying against a group of people and a kill maybe 2 of them, after I reload and try to kill them I usually get killed. The reason for this is that they're anticipating you to try to shoot them again so they're going to wait for you for a while. Once they think you left they'll probably go towards the area you where and from there you can finish them off.

6. Trick people. What I do is when I'm running by somewhere and I see a guy to my left I act is if though I didn't see them and continue running to the right, then after about half a second turn around and go to the left and kill them. This usually works if you see someone in your peripheral vision and you know theres no chance you can just turn and kill them.

7. Be on elevated ground. I don't need to explain this one

I like the Assault Rifle class I showed you guys earlier. I use that on domination with the killstreaks of: Recon, Blackbird, and Dogs. I have a run and gun style because I don't want to be stationary to the point where a guys kills me standing still.