Darkness is a useful ability; out of four heroes, three have it: Godric, Faelan, and Zehir. But I didn't use Darkness in the battle between Zehir and Biara; I simply didn't have time. I cast Banishment twice and Resurrection once, and that was it. But Darkness is useful in all other battles, so it's up to you.
I also have poor Attack and Defense stats for Zehir, especially Attack. After the fourth mission, I barely managed to raise his Defense to 15, but his Attack is only 11. So, no matter how hard you try, I don't think your stats will be much higher. For a mage, everything goes into Power and Knowledge. If you pump up his Attack instead of Defense, his army will be completely wiped out.

By level 22, it's already difficult to choose, because you can't upgrade skills later. And now, in the witch's hut, I got Darkness, so that's fine. And Zehir's Attack/Defense stats are generally 9-9, although these are base stats, without artifacts. Godric appeared in this mission at level 19, which is, I think, the level I left him at in his campaign. But, of course, he is stronger in terms of combat stats and also has Skilled Light and Chaos Development. Now I've saved the game at the moment of taking the second Turquoise castle and activating Faelan after visiting the Red Tree. Before this, Zehir had already defeated six Turquoise squads; how many are there in total? And Rutger wasn't among them yet.