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

yes, here it's still a game of luck anyway. That's just how the game is. Whether you get lucky with artifacts or not, whether you get the skill you need or not xDD. (For example, on small maps, unnecessary skills can ruin the whole game). And vice versa =) We once played against Hafiz on Underground Treasures (Hafiz already hates this map =)). Every time this map comes up, he has to play with this hero =). Anyway, I took Nur =P, and on the first level, I got Machines. So, the build-up went just as well as Hafiz's. (If the Oracle was a better hero, but I like Nur more =)). Regarding magic, you can level up Summoning and take Fire Warriors (when you're far away from your castle), or Light with Dodge if there are a lot of archers. (Darkness is so-so - it depends too much on the chance of getting it in the castle). But I like Chaos much more. (I can't give any advice for huge maps because I don't play them).