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4) This is what makes the map challenging. At lower difficulty levels, Vexel has a low level, so it should be easier there.
5) That makes sense. The probability of recruiting a Champion without a mentor is low, and once you reach the mentor, you can cancel the diplomacy skill – it's not very useful afterward.
6) It's difficult to kill him, but the losses should be recovered, so act more boldly!
7) This is not scripted. The AI acts on its own, so the order is, in general, random. However, it's best to take the castle near the garrison last, so it's easier to escape from the red heroes (if, of course, you plan to escape).