Hmm.... But even if we get a nice big bonfire, it's not likely to spread very far. And the hydra might be able to work out "Hey, no fire there! Go there instead!"
And that fast healing 15 basically negates any damage we could do to it, so any kind of fight isn't really an option.
In my opinion, running is the best option right now... Maybe we can lead it away from the river, double back and get across.
All of us at least match its speed, so at the very least, we can stay ahead of it on land. I think only Onesimus wears medium/heavy armor, though I'm not sure about that. But I think everyone else wears light or no armor, so we should be fine with the pace.