Shatter would work in this situation. The caster would have to use the targeted version of the spell and be high enough caster level (10/lbs per level), which shouldn't be too hard. The spell only effects non-magical materials, so this won't work against a magic shield, suit of armor or sword.
If the item in question is worn (or otherwise attended), the attendee gets a will save vs the shatter.
Antione brings up an interesting point that armor is usually comprised of many pieces, so you can go with a more "realistic" feel by having just the breastplate, pauldron or grieves shattered and then associate a comensurate AC penalty.