What is your favorite ultimate ability for that class?

Absolute defense is the most effective; a ranger won't be able to do much against such an army, even with their absolute luck, even though their troops will have a significant initiative advantage... but unfortunately, you can't stand against rune magic, or you need an army twice as strong as the dwarf's.
The Call of the Urgash is good not only because it can block ranged units, but with Nightmares, you can easily scatter all the defensive fortifications of the army, especially since the Call of the Urgash is valuable if the hero is Nimue.