Regarding the Necromancers. It's a good map. I just played it again today. To prevent these unpleasant people from reaching the main island, you need to scout the waters they jump across. And if they manage to cross, reload the autosave. The AI isn't that smart, and they usually end up jumping in a different direction, not going where they need to.
I had a different problem there. I killed everyone, but one of the Necromancer castles is glitching. When the hero enters it, nothing happens; the castle doesn't open, and it remains an enemy castle.
I'm trying to remember if this problem has occurred before.
(Okay, I found a solution below, I'll try it). :)