Heroes 5.5 WarGame Edition - Tactical Edition.

A high defense stat is not necessarily an indicator of survivability, unfortunately. There is magic, there is partial damage reduction, and there are ballistae. There is Orson with a huge amount of health early in the game. And it is this health that provides him with the necessary number of casts, not the defense of the Dancers. And not Archers, but Warriors.
Warriors are many times more effective than Archers in terms of damage output. It's been tested, don't argue.