Which magic schools should I choose for the late game? Or should I go for a specific artifact?

The academy army needs all kinds of magic!!! That's why I would choose the Spellbook... First of all, HAVE YOU SEEN ROCKET-POWERED RAKSHAS? (with vampirism + speed + onslaught + magic shield) Take a look! It's awesome)) Of course, the Spellbook doesn't have mass spells, and focusing on just one type is... not good overall. So, you need to adapt to a specific race: against Humans, the Spellbook definitely rules! They have training, so a powerful Puppet Master will easily defeat, say, Paladins, and that's almost it. Then, take on Angels, and Chaos will also be useful, as well as Divine Retribution!!! THAT'S IT!!! :)

Added 2 minutes later
The Spellbook is needed so that you don't fail with spells (in case they don't drop:()