Weakening - reduces the magic power of spells from a school of magic by 2 times.
Corruption - increases the cost of spells from a school of magic by 2 times.
I think that in the base game, 10-20-30% to HP would look very good; the question is in what order: Weakening first, or Corruption first.
So, a full base plus all basic perks (6 levels invested in this) = reduces the magic power of the opponent by 2 times for all schools except Chaos, plus increases survivability by 30% (which is relevant against Chaos).
For Orcs, Strength against Magic will be built-in by default (there will be no separate perk for this), meaning the magic ban will work initially (50% of the Orc's magic).
For Orcs, make 2 common skills for all classes (in addition to the racial one): these are Engineering + Resistance (a unique Orc skill).
The remaining 4 popular skills will be chosen by classes from 6 (Attack, Defense, Luck, Leadership, Logistics, War Cry).
There remains a 2% chance for the remaining 2 skills.