Which faction is easiest to defeat a late-game Dwarf player with?

They will be good even towards the end of the game...

Here, it's less likely. A Dwarf isn't much worse than a Town in Chaos, but Runes, increased troop survivability compared to Town... they'll win through attrition. But in the early or mid-game, I would try to take them out when they have a small number of Runes, using a combination of ranged units and Chaos magic.