He gives her an annoyed look. Good luck? Perhaps if he wasn't freshly adjusting to his cursed sword he would have let that slide, but Xin Mo loves it when he's insecure and lashing out, so it's absolutely encouraging him to assume that people think he can't handle this.

"It's only spiders," he scoffs. He doesn't need your luck, Gray! He looks her down with some amount of disdain. "I expect you could handle them alone too if you had to."