Boss Units are single individuals of massive size and power. The only shared characteristic of Bosses are their huge size and resistance to multiple forms of damage, otherwise their weaponry, defenses, and range are varied. Bosses are practically immune to all forms of melee attack and cavalry charges. Siege Engines, Mages, or a huge barrage of missile fire is about the only method of attack that can damage them.
The majority of bosses are heroes, since they are one of a kind and unique.