Gertrude Judith McGilliguddy has definitely never frozen under pressure in her entire life. That doesn't happen to brilliant people like her. No, she isn't at all surprised by this outcome and she was definitely just waiting on the opportune moment to spring into action.

It only takes her a moment to summon a happy memory once she's decided on her course of action. The others have the ice front pretty well covered, she supposes, so instead she summons her patronus. The deinonychus burst forth from her wand in a burst of silvery light and charges into the fray.