FireCool
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The thing is that lightning gets stronger with blood, and it's best up to the second blood upgrade, but even then, it's not very effective. It's better to use speed and slow spells instead, and the leaders should be pushing their units forward, as they provide much more benefit.
As for the speed of the shamans, here's the thing: if the AI goes first, you can use your shamans to cast speed/slow spells on their units. But if a player goes first, they will receive the opposite effect.
By the way, you still haven't answered me about the wyverns. I didn't understand your suggestion that shamans can debuff wyverns effectively.