Merlin didn't know what would happen when the power of will entered the Giant Dragon's brain. There hadn't been a fool brave enough to do so. Now, he felt like the fool.
Druids were a type of wizard. Even though they could use close-ranged attacks, they could only do so through self-enhancement spells. Wizards were also limited to their power of will. They usually only used it to activate spells, rune signs, hexes, etc. Only a few wizards would use their power of will offensively. This was because once their power of will was broken, there would be severe consequences.