Nature Structures

Of course, it's possible they won't go against a buffed Barbarian. But still, he isn't weak against the others. Firstly, he has direct damage spells, which are such necessary help for Nature magic. Secondly, again, confusion, which can slow down the attack of a level 4 unit.
Essentially, the Bard deals with units in battle. He can certainly hit the hero too. But for a specific showdown with the enemy hero—the bard can summon a praying mantis!