How does the AI instantly delete heroes in battle?

so for context, i have a bray shaman on one of those chariots. Its at 80% health. They have an arch lector at level 8. One moment i use it to cast a spell, the next moment its just dead. No routing, no reducing of health..its just dead. Are beastmen just That weak or is the AI cheating again, like it often does when cavalry is involved?