For both heroes:
0. On one hand, Summoning is useful, and we always take Dark Magic and Education. On the other hand, we forgo Mind Control and pump unnecessary skills from Summoning (wasting 3 levels). Personally, I don't take Summoning.
1. Dark Magic, Master of Pain is a must-have, Mind Control is useful if you don't take Summoning.
2. Summoning, you can do without skills if you don't take Summoning.
3. Education. One hero gets Mentoring. The other can be limited to Magic Affinity.
4. Logistics. One of the heroes needs to take it early to avoid being harassed by creatures. Find Path and Path of War are must-haves. Logistics is also needed for speedruns.
5. And finally, Sorcery. It would be good to take Shifting Mana and Secret Knowledge with Mana Regeneration, but only if you have enough levels.

Ornella should teach Arantir the spells. He should have many of them from the second mission. He needs Plague, Slow, Dispel, Berserk, Subdue, Vampirism, Unholy Word, Hive, Phoenix. A Disruption Ray is also useful for the yellow castle. Thanks to Mentoring and the Necromancer set, you can raise a lot of undead. About 70 uncursed vampires per week. One stack of vampires can clear almost everything without problems.
The final battle is with Arantir, so it's more important to level him up and collect all the stat-boosting items.