The easiest ways to kill people wielding riot shields (not including using kill streak rewards) are to stick them with semtex, shoot them in the feet, or throw a grenade behind them (hopefully right under them if possible).
Another way is to flash them or use a concussion grenade (I prefer concussion, because they physically can't turn to face you, whereas they can fluke it with a flash) and flank them and shoot their sides/back (or knife them).
Alternatively, wait until they lift the shield and shoot them (although I assume you've been trying this
and it's why you've been getting killed).
(When I figure out more methods, I'll edit this, but I haven't really come across too many riot shielders yet).
gaming.stackexchange.com/users/12087/Mr Smooth
I know of three ways: 9-Bang (Flashbang) or concussion grenade them straight in the face if you have Blast Shield and run into their sides or back, and then either put a knife or a bullet or two into them.
If they are in a confined space, then first throw the flash/concussion, then rush in.
If they are running, it means they are standing, shoot them at their feet.
If they crouch, try to flank them, still shooting.
The "still shooting" part is important.
Keep them nailed to blur their vision.
Throw a nade behind them.
Even if it falls far, this will make most fear and turn in the nade's direction, which leaves you with their back.
You know what to do..
If they are crouching and have their backs on a wall or a corner, then either SMAW them or lob a nade behind them.
Alternatively, if you are wielding a Riot Shield yourself, too, go charge them - may the faster flanker win!
Ok, that was five.
gaming.stackexchange.com/users/4182/DrFish
You have to have really accurate shots to kill a riot shield soldier.
One tricky way (which may be easy to most of you) is to aim for their shooting arm, and it is also good if you put C4 where you know they will be walking.
gaming.stackexchange.com/users/17380/Jo'nathan Tummins
Grenade behind them is what I have always liked :-)
It's simple and you can do it at a reasonable range.
gaming.stackexchange.com/users/6028/Rory Alsop
The way I prefer to do it is to throw a flash bang at them and when they're flashed run behind them an knife them.
In close corners I don't believe it's possible to have a real strategy if it's just you versus them.
Your best bet is to run to a more open place.
gaming.stackexchange.com/users/138/Earlz
Use a flashbang behind them and aim for thier shooting arm
gaming.stackexchange.com/users/37342/user37342
Knife, throwing knife to the ground right in front of them, flashbang, grenade (esp semtex), flanking These are the methods that I know of, use them wisely
gaming.stackexchange.com/users/320/Ritwik Bose
A high damage sniper rifle (AS50 or Barret 50cal) with impact proficiency can go right through a riot shield
and I've gotten the game winning kill on a fluke with this before.
Without impact the bullets will be absorbed so don't expect to shoot through a riot shield just because you have your sniper rifle equipped, if you have a different proficiency it won't work.
gaming.stackexchange.com/users/3655/RobV
Stab them, or jump and throw the knife at their head.
gaming.stackexchange.com/users/38366/R10T ŠHÎÈLD
I've gotten owned a couple times by people with riot shields, and it's almost impossible for me to get all way around behind them efficiently.  What is the method for killing this sort of player?
How do I kill someone if they are using a riot shield?
gaming.stackexchange.com
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gaming.stackexchange.com/questions/34811
gaming.stackexchange.com/users/13484/Sean Gallagher
Try sticking them with semtex or throw a nade behind them. You could also try a concussion grenade or wait until they lift the shield and shoot them. A variety of skills, strategies or weapons can be used in order to deal with the Riot Shield.
Using a grenade could be a solution to the problem. Using Flashbang or concussion grenade can also solve the problem. You can use sheer markmanship in order to kill whoever holds the shield. Holding a Riot Shield could be a solution. Using specific weapons such as Rifles and Knifes could serve as a solution.