The most powerful hero of the Dungeon faction.

The benefit of having Kifrah as a second hero is undeniable; she not only strengthens the main hero's army but also generates income. However, using her as the primary hero... Well, if you have a good starting position (3 dwellings), then, of course, it's possible and even beneficial. In other cases, she is valuable because she can level up and gain very useful skills, enhancing the army's combat capabilities rather than just using it as cannon fodder. Or, you can go for Empathy. That's also not bad. But Vaishan also has a decent start. Iranna can easily acquire the Mentor skill and set up ambushes. Sinithar allows you to save on knowledge points. They seem more promising than Kifrah.

Vaishan has a worse start; Stalkers are a weak unit and need to be upgraded, which takes 2 turns. Kifrah, on the other hand, can immediately capture a sawmill or mine. The Stalker growth is 7+7, and it cannot be increased in any way (in total, this is 196-210 hit points per week). Minotaurs have 6+6 (420-440 hit points), and their growth can be increased with a Pit. You can also quickly acquire Diplomacy, combined with high leadership, to throw many creatures into the Pits. The poison damage from Stalkers is almost unnoticeable in the late game; you can't produce many of them. Even if you have 400 units, dealing 400 damage over 3 turns from a poisoned creature doesn't seem very effective.
Sinithar isn't better in strategy; he has a Magic Arrow spell, while Kifrah has Lightning + Minotaurs, and in the mid and late game, he's not needed at all.
Iranna's start is even worse than Vaishan's. Fury units also need to be upgraded to prevent them from being lost. She doesn't have Chaos Magic at the start, and it will take 4-5 turns to upgrade them. And if you encounter Archers at the sawmill? Her usefulness in the late game is questionable; there are too few of them, 5+5 (160-210 hit points) per week, and they die too quickly. Still, this unit is number 1 in terms of being easily defeated.
With Kifrah, you can go for Empathy, combined with the Minotaur's Aura of Courage, or for Retribution; I don't even know which is better.
You're right about Mentor; that's where the tactic for Iranna is good.