Tactical techniques for conducting battles in Heroes of Might and Magic II.

In terms of defense, Barbarian troops are among the most vulnerable.

That's true, especially considering that the hero mainly focuses on attack (although not to the same extreme degree as in Heroes of Might and Magic III).
They are particularly vulnerable to Knights, but not as vulnerable to Mages – their level 4 and 5 units have more health than the others, and combat spells inflict much less damage on them. Destruction magic is especially effective against Knights, who have the lowest health stats.