Hydra uses WaveOfFear.
WaveOfFear is a Yellow enemy Tech used in Chrono Cross. In Hydra Marshes (Home World), Hydra uses WaveOfFear when weakened by battle. When cast, a spray of yellow sparks encompasses the party, decreasing their defense for the duration of the battle. This tech is used in conjunction with SpiritsUp.
It can be assumed that while fear-stricken, one lets one's guard down.