These are wizards. They can do, if not quite everything, a lot that we consider impossible or insanely hard. Maybe hydras have the magic property that they "stop time" nearby. For anyone near a hydra, time seems to freeze, at least until the hydra is dead. So if they can kill the hydra, the world pops back to normal with not a second past since the hyda attacked, but if not, they just get eaten.

This is a powerful attack/defense mechanism for the hydra, as the hydra's opponent must spend eon upon eon fighting it to win, and almost all give up before then.

At least, that is how I imagine Eliezer would write it if that kind of hydras appeared in canon and Eliezer had to come up with a reasonable explanation to it. Many things "don't make sense". How can dragons fly? They are to big to reasonably do so. But an intelligent author can "fix" things, and give a reasonable explanation.

Eliezer did that a lot to certain things from canon, and such is the correct attitude for facing a fictional thing you deem "impossible".