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And in general, a Barbarian can level up magic too... but why would you?
By the way, that's exactly what Barbarians do. Even though they still have very little mana... but still. They believe Nature or Life is something they absolutely need, at least levels 1-2. And there's another trick. They love carrying all sorts of scrolls (they basically have a warehouse of them). It's just that these provide their effect for half of the total mana, but for a Barbarian, that's not a bad option... especially if it's Haste or Teleport.
Personally, I believe leveling magic isn't mandatory for them... If there is time and opportunity, Tactics or Pathfinding would be better.
And in general, I feel calm about resistance, since the Barbarian also has an army. And my mage will definitely have plenty of room to run wild! :)