Help! I'm playing against human players with mages, and my hero is Orac, while theirs is Laslo.

Maybe they took a long time to start playing... in a long game, we will have an advantage. After all, the game has already started, and all the conditions are described. Answer this question, and don't talk about what might have been...

I answered as it SHOULD have been. Judging by the described situation, the author of the topic does not yet have enough experience playing against a human in Heroes 5. And I thought that it would not be superfluous for him to learn about a much more correct strategy, which would greatly help him in future games.
Now, about the current situation.

they started the game, and he started heading towards overwhelming power

although it's not his specialty, he developed Banish and Elemental Equilibrium, and I wanted to crush him with Phoenixes and Elementals, but he's doing something with Sorcery

Therefore, the opponent has all 6 slots occupied:
Counterstrike, Defense, Leadership, Summoning, Sorcery, Light.
With this build, he's not a knight, but a walking disaster. You can kill him at any time.

and he only has 20 mana, which is only enough for 2 mass spells

Considering your Light Seal, only one, or two if you double the mana. You need to stock up on several Dark spells, for example, Weakness for archers, preferably with Mind Control, Blindness, Berserk (remember that it doesn't work on paladins, but it works on knights of Isabeth), Dominate, etc. Not necessarily all of them, but several. Because of the build, he didn't become a warrior, and because of the 20 mana, he didn't become a mage. You will kill his hero at any time, because your build is significantly better, and he will not be able to resist the Dark magic