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Hello! That's an interesting question. In general, I think it's better to apply positive effects to your own units rather than negative effects to the enemy. For example, if you apply a speed boost, you'll move faster. As a result, you'll deal damage to the opponent's units earlier, which means you'll weaken their unit, and a weakened unit will deal less damage to you. But if you slow down the enemy, you'll just delay the inevitable, and that's all.
