Keep in mind that certain villains, like Kel'thuzad, are nothing more than "mere" heroes from Warcraft 3.
A lich hero in warcraft 3 can probably solo a couple of footman/grunts/whatever, but 10 or more would definitely overwhelm them.
For most of the legendary foes we had significant help, and for regular villains (like anub'arak) we simply overwhelmed them since they are just regular heroes from war3.