Warlock – “Cultist” [Campaign 4, Mission 3]

Yeah, mages are very slow, so logistics are essential for them. On this map, Leto seems to be the strongest enemy; I had the most difficult battle against him, and I lost a lot of troops to his plague and magic. In general, in each new battle with him, he cast the plague differently; it could affect the witches, or it could affect the harpies and assassins. You can only figure this out in advance on the battlefield by looking at who he casts the spell on, loading a save, and changing your tactics.
I'm not sure about applying the plague to dragons, but it should work. After all, the plague is not an offensive spell; it is a type of curse, like weakening or misfortune.