Oddly enough, by biting him. There was an issue of Amazing Spider-man by John Byrne a while back that had Venom eating a chunk of Sandman. Sandman lost physical integrity after that. In a recent issue of WatX, Beast eats mini versions of Iceman in a fight.
Beast bites a chunk out of Sandman and wins.
Ok, it wouldn't happen that way since both events seem to be PIS, but it'd be funny if it did.