The sorcerer won't actually hurt you, but if he freezes you, Graham is vulnerable to other enemies. In a few seconds, Graham will thaw out and be able to move again—hopefully without any other encounters.
The best way to deal with the sorcerer is simply to avoid him. He doesn't move very quickly and the few places he shows up are easy enough to move quickly though.
It is said that the King was fooled by a sorcerer who tricked him out of the Magic Mirror in exchange for giving his wife fertility. But the sorcerer ran off without fulfilling his end of the bargain and the Magic Mirror had not been seen since. Although it isn't mentioned, it's possible this sorcerer is the culprit.