Best way to kill someone with a shield is to sidestep around him and stab him. Even if he removes his shield he will still be open to a stab so you win really unless he can bash you with it twice and humiliate you completely.
Failing that something heavy like a nade launcher at the feet or side of them usually works. Remember if someone comes at you with a shield and they are stood up, blast their feet it'll get em quite quickly.
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