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

Here's my advice regarding Razazak. Level up Chaos Magic and Battle to the Last, and place them on your mages for defense. When attacking mages or gremlins, cast Wall of Fire on the golems (if possible) and run around on it. Try to place multiple walls in one spot. You'll be amazed! And finally... if you're cornered, use Armageddon. The golems will be resurrected under its effect, and the rest will burn with hellfire!