What about if the Gargoyles masquerade as Statues, and when Merlin attempts to walk into the Sanctum, then the Gargoyles will spring forth. Gargoyles should have a rending, tearing attack that does high damage with a high attack rate. Despite being immune to Merlin's Army, Magic should be able to make short work of it, as its magic resistance should be quite low.

HP: 150
Size: About the same as a Boulder Monster
Damage: 8-10 Damage, Attack Rate x2 of Normal
Speed: Medium-Fast

Special: Takes no damage from Merlin's Army
Spells do 3x Damage to the Gargoyle