Which faction poses the greatest challenge for the Dwarves?

I only have 1 standard hero - Naadir, and he levels up in a standard way: 1. Darkness 2. Summoning with Mines 3. Leadership or Defense, if the previous skill doesn't work out 4. Education 5. Sorcery At level 23-25, IMO, this is already quite good; the Necropolis army will be so strong that 5 Fireballs won't save you. Against the Necropolis, the Dwarf will definitely be useful. Runes don't seem to remove the Plague effect.