The Hydra is a powerful monster that has the potential to kill PC's.
In playtesting one group ran away from the Hydra to area 15. They then trapped the Hydra using the shields. Top marks!
Another group of 5 level 3 characters attacked the Hydra and used non-lethal damage to knock it unconscious. Also good.
However, a group of 6 level 2 characters decided to split the party. 4 went to area 10 and 2 went to area 7. This had TPK written all over it. When the 4 PC's saw the Hydra they charged towards it despite the description of a huge creature with 3 heads eating another huge creature. They'd also been told earlier that the specimens are all powerful monsters and that's why the shields are needed. Of course they also walked over the dozen dead bodies at area 3. Despite this they charged the Hydra. It was a close fight but two of the PC's died unfortunately. The Hydra was killed by a bow shot (which can't be non-lethal damage) so no magic items for them as Raven was not impressed. The other 2 PC's triggered room 7's undead horde but smartly disengaged and lived to fight another day.
The Hydra can be used to setup a tense chase scene or an exciting battle. But as it says on page 6 "The life of an adventurer is a dangerous one and there is a genuine risk of your character dying, particularly if poor decisions are made." As Game Master you can do your best to present the adventure information so that the PC's have a good picture of the difficulties they face. However, what happens is up to the players :)