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Regarding Hydras, I'll say this right away – no matter how many targets they engage, they will only drain life from one unit, the one they are attacking.
And I usually either cast Vampirism on the Red Vampires if the game has progressed to that point, or on the Mistresses, which is more effective. Because while invisible, they can survive the main exchange of units, and then reappear with Vampirism applied, when there's not much left to fight them with. Besides, they can be effectively contained by a Phoenix in a corner, protecting them from most of the enemy army. But you can't hide Riders like that. And their strength lies in their momentum. And they can be blocked.
Minotaurs are also quite formidable. However, Vampirism reduces their morale. And Minotaurs don't have great initiative, which will become apparent when Vampirism is applied.