From Guild Wars Wiki
11 Fire Magic
- Hydras tend to initiate combat with Area of Effect Fire Spells when players enter their aggro range. These spells can cause especially high damage to a party's henchmen or heroes, who tend to cluster around the lead player. Parties made up of grouped players may wish to spread apart to avoid presenting densely clumped targets to the beasts.
- Hydras may also sprint through gaps in the battle line to attack rear-rank characters with both PBAoE spells and melee attacks.
- Where possible, it is best to separate Hydras from the other groups in the area, and fight them under more controlled conditions.