"Let's do it." Because Byleth is quite out of constructive plans for these things. An enemy that he has had very little time to study or come up with a strategy.

With Lahabrea working on the spell, Byleth sets about keeping the creatures off his back so to speak. Managing to keep them at a distance is tenuous at best, but he does it with the help of his acidic breath weapon.

The decisive words come from both his confidence in Lahabrea's magical aptitude and the urgency of the situation at hand.