Which faction poses the greatest challenge for the Dwarves?

Are you referring to rushing the Dwarf? If so, I agree that easily obtainable machines + enchantments do the trick. If it's a late-game scenario, things don't look so good for the Orc. The Dwarf will have Inga (as the second hero), and with her, all the runes. And it's not a given that the Orc will guess correctly with the Silencer (he might end up with a Chaos Archmage and a Trickster).