Auto-translated
I'm playing as Helmar. I'm focusing on Light magic because of his ability. At level 15, after casting mass speed or mass blessing, almost my entire army benefits, and they also have the punishing strike ability, which is very useful for gnomes. ;) This saves a lot of time by eliminating the need to cast different spells on the army, and with the 2% chance to learn spells, I think it's crucial.
Soldier's Luck is definitely one of the most useful skills (many units have special abilities that trigger).
I don't invest in Runes. The initial ones are good, but the others didn't impress me much.
Right now, I'm deciding whether to recruit Harpooners or Pikemen. As ranged units, they're both not very good. But the crippling strike is quite useful against fast enemy units...