If you're still getting used to the game, a few tips:
Use a sniper if you want to sit back more and camp. Barrett .50cal is nice for one shot kills - I got into MW3 using this gun. If you want to move around a bit more, I suggest the ACR 6.8 with a silencer and the focus proficiency when you unlock it.
Kick (or attatchments)
Heartbeat sensor (or/with extended mags)
MP9 w/ silencer
Bouncing Betty or Claymore
Portable Radar, Flashbangs, or Scrambler.
Keep back to the farther parts of the field. Take your time to move. The heartbeat sensor will help you pick up anyone not using assassin that's creeping up to you if you choose to use it, and you can then switch to the MP9 to finish them off.
Focus (or attatchments)
Silencer (and extended mags or the ACOG scope)
Stinger (or FMG9 Akimbo, if you really need a boost. But I suggest Stinger.)
Semtex or C4
Flashbangs or Portable Radar
This you can move around the map a bit more with. Check your corners, check any pathways you cross. If you're playing Domination, use C4 and plant it in a spot next to the capture points you own in case you start to lose them - usually they do not notice the C4 and you can just blow 'em up for a nice double kill or so when you notice you're losing the position. And you defend it, whoopee! Portable Radar can help you keep an eye out on certain areas in the map, while your silencer and your perks keep you invisible.
Your strike package should probably be support, in this order: Ballistic Vests -> SAM turret -> EMP/Juggernaut/Escort Airdrop (which you choose is entirley up to you, and this is only a suggestion).
Hope this helps, enjoy!
p.s. Get used to higher sensitivities in order to keep a more wary eye out, but not so high that you're zooming past where you want to look.
Why the need for the shield anyways? Thats the main problem with your KD that your worried about. In MW2 it was nice because you could wear it on your back as a turtle shell but this time around it slows your pace down so much.
Other tips- Stay on the outskirts, make your way around the outside of maps traveling clockwise or counterclockwise. When you get to a point on a map where survivability is low (like the front doors to the mall in Arkaden) dont risk it, turn back and go in the opposite direction. Or just patrol up and down one side of the map like on Mission.
Almost ALL 10 of my classes have Marksman accept SMG and Shotty classes. Youll see people you never knew were there. Its NOT just for snipers. Try Stinger as a secondary and Trophy for equipment so you can trade up Blindeye. Predator comes in quick pop out the trophy and youll be untouched... most of the time. works with reapers too.
Right? the main point of the shield is to get shot at. youll never go positive like that. Ive never seen anyone go positive using a shield all game. But besides that, your shield class is messed up. Specialist package? and your first perk isnt selected. I would put blastshield on over assassin. people see your shield and they grenade or flash the crap out of you. You might wanna switch to support because i cant imagine youll get many streaks with the shield.
If you really want to stick with the riot shield, you might want Overkill on and have a SMG or AR, preferably PP90M1 or ACR 6.0
whenever you're sprint and come up to someone, crouch instantly, it stops you sprinting, allows you to pull your gun out faster (even with quickdraw) and you're a smaller target for them to hit.