Snape's pulling of his punches is intentional; he's upset, he's angry, but-- when confronted with an angry, upholstered reptile -- he's wishing he'd gotten serious (Ahem) and downed the Dragon's breath potion instead.

"ARE YOU DAFT?" That was never in question, Snape. He's firing off stinging hexes at the thing as it lumbers about. Hopefully, it can't dodge... because he's gotta do that, manage a shield charm, and get the right potion out of the case with a double-team.

Hopefully the boils are making it hard to see over there, so Snape's not too glaring of a target.