Ok. Yes, the Black Knights are most powerfull units on the game.
I found that one stack of Black Knigts is better
than 3 stacks of snakes (green, red, royal)
with 2 Snake rings and 2 Snake boots. Or simply Black Knights are better
than any artefact-completes.
As for the Mage, the spell
slow is very effective. And the spell Summon Phoenix,
if anybody is lucky enough to find it somewhere at the begginig of game (first 3,4 islands).
On impossible mode, the Droids are essential, at least in the beginning.