The eternal question: which heroes are better, mages or warriors?

The force field solves everything! They will just skip their turns! :)

Is this like the sword walls in Heroes of Might and Magic V? Damn, I forgot the property of this structure. Earlier, when I just started playing Heroes of Might and Magic III, I even thought it enhanced ranged attacks. :D

Once, I killed 25 black dragons on cursed ground with 3 Archdevils without any magic at all!!

It's quite possible. Archdevils have a retaliatory attack. It also depends on the presence of obstacles on the battlefield.