Note 1: If you don't take down the hydra quickly enough, it might survive. To be honest, Yan'shi is hard to kill; I wouldn't be too surprised if he's currently freezing the hydra from the inside with his passive ability. But you need to prepare for the worst, of course.
Note 2: The hydra is currently attacking Antaeus. I highly doubt that mana shield will help against being devoured (by the way, it didn't help Ples). But with high agility and a bonus, 88 evasion... He needs to attack its head each turn and withstand a total of three attacks. If you're ready for that, Heroist, then let's go. Visu should probably continue burning it, but he knows what he's doing.)
Oh, and one more thing: in general, you could have set the hydra on fire and tried to move away, hoping it would slowly burn (occasionally adding more fire). But that also has its drawbacks; its speed is unknown, and Ples is losing his last chance to escape from the monster's belly.