Heroes 5.5 WarGame Edition - Tactical Edition.

As a backup solution, if it's not technically feasible to implement something else, we could use the option of having tiers 1-3 for the "Sworn Enemy" skill as the base skill, and provide other types of bonuses for advanced and master levels, similar to the Orcs' "Distortion" (in this mod). For example, these could be standard bonuses like +0/+1/+2 to initiative or +0/+2/+4 to attack. I think we should implement this first, and then try other solutions. In that case, however, the "Avenger Guild" would have to be replaced with a racial bonus of +10%.

P.S. And even better, if different heroes have different tiers for their "Sworn Enemy" skill, it would be difficult to counter all the Elves at once by avoiding tiers 1-3.

This approach doesn't work for me because there will be many armies, and specializing in countering just one type is less effective. Imagine: the opponent has 2-3 producing cities of different races, you specialize the "Avenger" against cavalry, but another army with wyverns defeats you.

That is, units of levels 4, 5, 6, and 7 will never be able to become "Sworn Enemies" - this is not very good.
And in general, if you initially assign "Sworn Enemies" to each Elf individually, it could turn out that the choice of hero comes down not to choosing based on specialization, but based on pre-defined "Sworn Enemies" that cannot be changed later. For example, if I'm playing against a Necromancer, I naturally won't be interested in a hero whose "Sworn Enemies" are zombies and necrolizards; I'd rather take the one whose "Sworn Enemies" are, for example, a vampire, a lich, and a skeleton - you'll agree, there's a good chance of dealing double damage to key units, which is more valuable than the hero's specialization bonus (besides, it might turn out that a specific hero's specialization and "Sworn Enemies" are more interesting than another hero's). If we go down this path, we need to distribute the "Sworn Enemies" so that these sets are equivalent - for example, one hero has a lich, ghoul, and skeleton among his "Sworn Enemies" from the undead, another has a vampire, necrolizard, and zombie, and a third has a lich, necrolizard, wraith, etc.