Unfortunately, these are the conditions of the agreed-upon game. And, in principle, I want to kill the Academic in the late game. I already tried Laslo, even gathered 1800 zealots, but he went into dark magic instead of light magic.

Choosing "darkness" against the Academic, especially with such a large number of your own troops, is not the optimal solution. A knight hero with a classic build would be better: "light," luck, leadership (empathy), and defense. The last skill could be attack (BB) or education. Since mages have an initiative advantage, I would choose a white-grade castle – their units are tougher. Also, knights and militia can inflict a stunning blow, which will at least reduce the opponent's pressure. With Laslo, knights will stun with a high probability. Despite all this, it is more profitable to train monks than swordsmen. However, the special ability of Myv is easily countered by the Academic: he casts spells more often.