I saw the CoD part and almost got angry, but then I saw that you were a sensible person.
So I'll give some tips.
Cloak helps alot, but don't overuse it, you're gonna need the energy for other things, such as running, jumping, and armor. Always flip on armor mode before shooting an enemy, unless you're low on energy.
If you're going to go for an armor class, I'd suggest the Mk. 60 as a primary, and perhaps if you favor a shotgun using the loadout pro module to carry a marshall as a side arm.
I presently use SCAR or Feline as weapons, as they seem to get the job done well.
Though a select few may work, I'd highly recommend abandoning some CoD techniques, such as quick scoping or relying only on melee.
If you're playing on a vertical map like skyline, and you want to get some perspective on your battlefield, you should make your way to the top of the greenhouse in the middle of the map, and scope out the map with the visor, tagging any enemies you see. And camping will NOT get you anywhere, since this game is focussed around moving around alot, and you will get assassinated by some stealth guy by camping.
Now there's the laser sight attachment which helps alot with guns like the feline, but it trades a bit of stealth, so be wary, and know your tradeoffs, since it makes the gun extremely accurate but has a laser that shows the enemy where you are. Also, headshots can help alot, so aim for the head. Always keep moving, I cannot stress this enough, high mobility helps you get the drop on enemies, letting you get the first shot.
To help your aiming skills, you should start a private match with just you, and just practice shooting objects, moving around, jumping, and try to keep your gun aimed at it.
And grenade is double tap Y if you're on Xbox, PC is pressing G, and PS3 is double tap Triangle.
Hope this helps.