Seeing Molly, seeing something threaten her, lit a fire in Harry. And fire was something he knew to his core. Drawing strength from down deep, he lifted his walking stick come blasting rod, and spoke. "Fuego, Fuego Maximus!" The fire that boiled from his will and his word started as a thin funnel, but built as it flooded toward the creature like a roiling avalanche gathering snow. The fireball that hit the creature was nearly it's size and crackled with power.
In it's wake, Harry slumped against the side of the alley, feeling a wave of weakness slam into him that nearly sent him to his knees. Despite that, he fought to keep the alley before him in focus, and to stay on his feet, rather than fall over as his body was telling him to do right now.