The hydra does not get a free pivot, models when charging get one wheel, and that wheel can't be more than a 90 degree turn. Which means a flee from the spearmen of more than 3 or four inches would possibly take us out of the legal charge range of the hydra.
Am I thinking about this correctly? Only an option because we're so close to the hydra and that we're pretty far to its line of sight to begin with. Maybe we should have planned to flee from the spears...
Personally, I was thinking hold, hope the spears fail their charge and see a comet come down. But the plan isn't working this game (4 tokens)
But I think this is a good rule question that could easily come up in a lot of games, and it might be one of those times we should have fled with a solid stubborn unit.