The reason for the Sorceress' success is that she isn't herself. The person that everyone knows as the Sorceress is in fact nothing more than a puppet, the Sorceress' real body being a statue hidden away. Due to this anything that is done to her is in fact only done to her puppet. Magic, weapons, and spells all effect the puppet without harming the Sorceress. Even if the puppet is killed or destroyed the Sorceress needs only to hop into another body, some times that of the magic user themselves. Even the most powerful battle mages can't compete with someone they can't attack. And since the Sorceress' true form is secret, even to herself, no one knows the correct target to stop her rule.
After so many failed attempts to stop her, many powerful Magic users gathered together to find a solution. They decided that since they can't destroy her they would banish her. They cast an enormous spell upon the Sorceress' entire garden cutting it all loose from time and space. Her domain now floats free, drifting here and there, instead of a hazard for one place forever she is now a menace to all places, but for a short time.
Her Garden can be any where at any time, shoe horning its way into its new homes. Looking for all the world like it always belonged there. However the spell wasn't perfect and many paths to the Garden have become stable, lasting for years or longer. So beware, any long stroll through the woods could find you in the Sorceress' world