I'm not sure why the Spell of Mastery is so hard in the game, but I think it's too complex in it's current form. Not only do you have to spend a great amount of research to get the spell but then you have to build 4! (maybe it's 5? I don't remember exactly) huge towers that take 40 to 100 turns in most cities and then control 1 of each shard as well as then spend 25 turns to win the game?
That's just a bit over the top don't you think? I would think one tower would suffice and it would need to be build in your capital city. I would also recommend that your hero has to be present in that city and is immobizied while the spell is being created. Thus your main hero isn't available for any last minute battles with the AI as the AI attempts to disrupt the spell.
JMO. I love the idea, but the way it is currently it's just too hard.