Found in Ash Lake, inside the hollow tree. When entering the tree normally, the spell will be unreachable. To reach it, the player must walk on the branch outside (the one with a Basilisk on it) and stop halfway, turn the camera, and fall down onto a lower branch.
Great Magic Barrier greatly increases magic resistance for its duration. Great Magic Barrier is considered a personal buff and thus cannot be used in conjunction with other personal buffs, such as Power Within, Flash Sweat, or even Green Blossoms etc.