It's doable but like, why? Sylvania had a tendency to spawn out zombie hordes that would occasionally march out to start killing everyone or cause some other Vamperic disaster, meanwhile Melkhior while really bad, is mostly just hanging out in his tower?
It's probaly very possible for us to kill him, but that's gonna cost priests, mage's, several soldiers and ton of equipment and so forth. A massive cost for someone that isn't actively causing trouble. Never mind the fact that Forest of shadows probally got several other, Hero-characther just lurking around there, that's gonna be drawn by the fighting.
The Forest of shadow is a huge problem at the moment, but on the list of actively causing huge amount of trouble to the empire it's probaly not that high