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I don't see the point in focusing a hero on a specific skill (unless it's a Special Ability). I level up heroes like this: I look at the skill wheel, and if the skill I need appears, I take it immediately. If not, I take the skill that's most useful. The exception is when I need to acquire a rare skill. Also, sometimes you need to act differently if a particular skill is needed urgently. But generally, that's how I do it.