She has a morale bar, but no morale...morale runes do nothing for her, and neither does being attacked (to gain morale). Her bar never changes, and her skill never changes. So it must be a graphics glitch.

I have tested her with skill attack, physical attack, and magic attack -- none of these seem to change the power of her spells in any way. Hit Rating seems to have no effect either -- I have yet to see Herja miss her spell. It seems to be a constant 20 morale gain to your side, and 5 loss to the enemy heroes regardless of gear.

Since my first round of tests indicate that no "damage" runes or staff effect her, I guess your only option is to put tank runes on her. You wouldn't need to level up the staff, either -- just upgrade it for the 5-set bonuses.