A ball pein hammer will work on metal armor, but all you need for denting plastic is a lighter and the rounded handle of a screwdriver (or a ball pein hammer ). Heat it up from the back/underside (avoiding direct contact of the flame). While it is still warm, push the handle in lightly and with a twisting motion (to avoid sticking). Make sure you don't overheat the plastic or you'll end up with a hole instead of a dent.