That's definitely Kubo's voice but there is nothing even remotely 'definite' about the thing following him and it looks like he can't bring that thing down himself.

"Kubo!" She needs him to know she's there so that he doesn't turn his instrument on her out of fear, but it also robs her of her opportunity to use any kind of stealth. "Get behind me!"

But if she aims her bayard between it's legs and hooks into the rock behind it, before whipping it to the side. Banking on the monster to trip and fall is...actually a really tall order but if this works it'll buy them an opportunity to kill it.