to get those 8 ghosts I had to accept a hero offer with 4 ghosts (around 3000 gold) and produce and vector ghosts form 2 other cities, and than chase a hero. it wasnt that easy (although my opponent helped me
of course it is good to have a lot of ghosts. but it is also good to have a city producing devils (to cancel dragons and medusas), a city producing grandarchons (to cancel heroes) etc. but I think that ghosts are a bit to fast - 18 move would be better, and they could be a bit more expensive