Barbarian magic suppression skills...

And one more thing: your friend made a big mistake by taking Wolf Riders instead of Executioners.

Actually, this move allowed him to last longer in the first battle scenario;)
Because Executioners would have caused a complete bloodbath in his ranks. Wolf Riders are not that aggressive. And with their aura, he increased the initiative of the Centaurs so that they would go before the Imps.
I adhere to this rule: if there is a chance that the opponent will develop Subjugation or Berserk, then Executioners are potential killers of their own army.

So, I'm leading to the fact that even if he had Anti-Magic (Dark), it wouldn't have saved him. And with Anti-Summoning, he might have even won - there wouldn't have been such a terrible Phoenix, he would have ignored the subjugated Wolf Riders and tried to eliminate my army - fortunately, it was smaller than his...

In conclusion, I will say that it will be much easier for me to play against a Hero with Summoning magic than against a Hero with Dark magic. Therefore, I develop Anti-Magic (Dark), but not Anti-Summoning.

If the enemy has both of these magics, then Anti-Magic (Dark) will not help, but Anti-Summoning will be very effective...