Oh well, so much for the surprise attack.
Still, with Hilary's unexpected adoption of the persona of the Berserker/Tank, Art and Kat can let her act as the lightning rod for all the goblins whilst they sneak in, ignored in the resulting chaos.
GOBLIN GUARD: "Hey! Where are you two going?"
KAT: "Don't ask stupid questions you idiot! There's centaur berserker in the Great Hall! Stop her before she accidentally breaks something important!"