Tips for the riot shield
-You're like the Heavy in TF2. You move slowly and have no quick and simple counters in a 1 on 1. However, you will die a lot if you have nostrategy, because the enemy team outnumbers you and is much faster.
-Keep the enemy in the center of your screen. This takes practice when you get close, but usually guarantees the kill
-Don't use it on wide-open maps
-Know when to crouch and stand. Standing means your legs are vulnerable but move faster, so the enemy will only hit you if they aim carefully. Crouching means you're mostly invulnerable to gunfire in one direction, but move slowly.
-Stick with teammates. Their run-to-behind-and-knife, or grenading, sort of tactics don't work so well when someone is shooting at them AS they think about how to deal with the shield guy.
-If someone has FMJ, switch class. As far as I can tell, it goes through the shield.
More general tips
-Weigh your killstreaks based on how many kills you normally get. I'd say if you get a 15 streak, normally you'd win anyway. Most of the time the simple things like UAV and carepackage are enough.
-If you hear about an enemy UAV, take a few seconds to see if you can spot it in the air. A FEW SECONDS (lest you get free-killed)
-At least one of your classes should be good at taking down aircraft, but be sure to time your attack on it so it can't fight back. Or, if attempting to kill it yields bad results, just stay inside.