Two sisters.

Fighting against such overwhelmingly superior AI stacks in an open field at the beginning of the game is almost suicidal. A Pyrrhic victory over one AI won't allow you to win against subsequent ones. The Knight has little mana to restore defeated stacks. Magic isn't a natural skill either. Under these conditions, light magic in an open field won't save you strategically. By the way, is the Leadership Crown a "built-in" artifact on the map, or am I just lucky?

Added 6 minutes later
By the way, Divine Retribution, a skill of one of the sisters, isn't very useful in defensive battles, and the Knight's native skill, "Preparation," is even less so, as there are enough troops anyway, and on this map, there's plenty of gold. The Dwarves didn't rush to return the outposts captured by the scout in their Dungeon.

I don't quite agree with that. There wasn't a single Pyrrhic victory, except for the battle with Rolf. But there, it was definitely not a good idea to fight in an open field – I replayed it later in the city. With the other Dwarves, there were almost no losses. The Ring of Sar-Elam solves all the problems with the lack of mana. The Lion Set allows the hero to move frequently. Dodge protects against 70% (if I'm not mistaken) of the damage, and Stone Skin adds +12 to defense. With an upgraded defense, where Endurance - Impregnable - Readiness - Battle Fury and Tactics are, respectively, the Militia and Armored Guards are simply incredibly durable soldiers. The damage dealt by Erlang's Archers is easily healed by the Archangels. By the way, Natalia cast Resurrection for 590 (~), which isn't bad. To be honest, playing on Veteran, I had no losses at all until Rolf. Except for maybe a dozen militia.
There don't seem to be any artifacts "built into" the map, except for the Lion Set and the Tome of Dark Magic in the graveyard.