Which faction poses the greatest challenge for the Dwarves?

Yes, it would be interesting to see a battle between a dwarf and an orc. But I wouldn't recommend using the Chaos Orc, honestly, as it significantly reduces damage output due to its Fury ability. However, summoning spells (wasp swarm, hive) have a greater impact on initiative, and there are other useful spells available: phoenix, a shield that will protect the already strong dwarves, elementals. But that's just my opinion.