The most useful monster.

If possible, you should cast Forgetfulness on the Beholder to prevent it from causing too much trouble. And while they're moving, use your hero to dispel any negative spells and finish them off with archers.

This is, of course, the ideal scenario, but it's not always possible... unfortunately. They certainly pack a punch. It's unpleasant to encounter them in an open field. And as a rule, they guard various resources and skill schools.