Judai readily backs up. Since they're in tunnels, there's not a lot of places to escape; his—their—options are limited on that front.

"I'll be quick, but what magic are you going to use?"

He keeps an eye on both Trahearne and the creature as he says it; no sense in losing track of what his companion's doing. Normally, keeping an eye on his partner's movements would be all he needs to know, but that doesn't apply here, where he doesn't know what said partner is capable of.