Yeah, mages are very slow, so logistics are crucial for them. Letos, in my opinion, is the strongest enemy on this map; I had the most difficult battle against him, and I also 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 seeing who he casts it 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, for example.