The best way to kill a Cyberman is with gold. Unsurprisingly, their greatest enemy were the Vogans, a race who live on a planet made almost entirely of the stuff (or at least they did, until the Cybermen blew it up out of spite).
During the great Cyber Wars, scientists invented the Glitter Gun, a device capable of firing gold dust into the breathing apparatus of a Cyberman, suffocating it. It must have come in handy at Christmas...
In later years, script writers became somewhat over-reliant on the use of gold as a weapon against the Cybermen. By the time of Silver Nemesis, it seemed that anyone with a bag of sovereigns and a catapult could take one out.
Another spectacular way of massacring Cybermen is the Raston Warrior Robot. A sleek, silver killing machine, the Raston Robot despatched an entire Cyber-patrol in seconds during The Five Doctors.
You can also kill a Cyberman by melting the plastic bits in its chest unit with nail varnish remover or other strong solvents. Or running over it with a steamroller, probably.