She said to go and she'd be behind him, and Garrus obeyed. Obeyed until the familiar rhythm of her footsteps falters, the sound of a struggle coming closer. He stops, he turns...

Slams the butt of his crossbow onto the skull of the creature attached to her foot. It's an awkward, desperate gesture, hoping to knock it off, at the very most. Drag both of them the last few feet across the threshold of their escape route.

Long limbs don't fail him now - grabbing for Shepard's arm to haul her the last foot forward.