If the armies are large and the hero is truly powerful, you can send in some expendable creatures to harass the defenders... They don't live long, but they effectively block the archers!!!
Thanks to this feature, you can also use them to absorb enemy attacks (since they're not your own, you won't mind losing them) and pass through without losses. It's more complicated for Necromancers – you have to restore them one by one.

Regarding the units – all of them are good, except for the Imps, who look awkward while running... Unlike other factions and their units: vampires, gargoyles (!!!), unicorns.

P.S. And didn't the Nightmares pierce a hole in the pot in Papa Carlo's room with their horn?:)