Geyser spam from enemy hero - Any way to prevent it?

[Ryastar]

Zag-Zag was easy for me (normal warrior). His army consisted mostly of peasants, and I had evil beholders. He got off a geyser on the first round, but I made sure to kill everything except his biggest stack of peasants. Evil beholders' ability triggers 100% of the time vs level one units, so he had no ability to cast spells, since I hit the peasants every turn from then on. I used the time to have my paladins res everything with one prayer, dig up chests, and get some points towards some medals.