Which faction poses the greatest challenge for the Dwarves?

The Ice Shield should bring good luck. And if you take Anti-Chaos, then, according to Murphy's Law, the Dwarf will end up on the Light side :( To take a Stronghold, you need to be sure that you won't be killed before you can develop it (depends on the map), as you take machines and lose your influence. In general, taking a Stronghold without a mentor is a risky business: they might offer a Ballista or a Tent when your skills are already maxed out, and that's goodbye to the Stronghold.
Of course, a lot depends on the random events. Only Immovability is so unpredictable.