You meet the Hydra in chapter 4. This boss is made of three heads:
- Green is immune to electricity and susceptible to fire;
- Red is immune to fire and susceptible to water,
- Blue is immune to water and susceptible to electricity,
Finn is the best character to fight this boss. If you've developed this character well, he will have all three types of attacks available. The other character should be Rubella. She can heal Finn or herself. If you opt for Aurora instead, make sure you have healing potions.
The green head can paralyze your characters, which prevents a single attack of yours. Focus on the green head first. Once you get that out of the way, nothing will hinder your fight.
Kill the read one next. She can damage both your characters. It's not difficult, but you've got to keep one eye on your health bar. If three heads attack one by one, you'll have no chance of healing. Don't linger - if your health drops down to 50%, use the potions. It's better to forfeit one attack than to die, right?