Shionis
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Agrail is already quite optimized and could potentially reach level 24 if you're lucky: 3 useful skills by default +2 witch huts.
Zephyr: similar to Agrail.
Railag: by default, he has an unnecessary defense, but theoretically, it's possible to reach level 28.
Markel: similar to Railag - the completely unnecessary leadership will hinder us in obtaining the desired skill.
Isabelle: similar to Markel and again, leadership.
Faydaen: impossible, the tactics ruin everything. But in the last mission, we'll get a hero with this ultimate ability, and you'll be able to try it out.
And use the skills wheel version 2.1 for the original game version 1.6 - I messed up by downloading 3.0.
In general, I think the skill tree is terribly designed. Builds are not flexible. A chain of skills is tied only to 1, and not to all 3. And this ultimate ability - it should have simply been tied to the level (respect to the variant).