Seth has just managed to get his aim right when the barrier breaks. Seth tries not to react, not to let the chaos distract him. If he can just get a clean shot, it'll hopefully be over. Gabe can take care of himself, the dude can teleport, hell, he can probably fly.

Except then he hears Gabriel's cry, and he realises that for some reason he can't. Perhaps he used up all his energy with the barrier. Making a snap judgement, Seth lets the arrow fly - not towards the eye, a small moving target easy to miss - but towards its chest, where it's guaranteed to hit.

Seth's already running to Gabriel's side when the arrow pierces the lizard's hide. Hardly enough to kill, but it does make Godzilla recoil with pain (or annoyance, who knows), giving Seth time to slide to his knees near the edge, looking down. "Gabe!"