Hmmm... Against Ulm I would go with a Water9 Pretender with atleast 6 dominion. If I made contact with Ulm quickly I would rely on my high defence and very fast Serpent Dancers together with Poison Slingers. Ulm troopers got good protection but lack in Attack so my forces would be able to poison them very nicely. Empoisoners would wreak havoc on the slow and low magic resistance commanders of Ulm and Sauromancer would outresearch them in a hurry, giving me very early access to Shadows and other Etheral undead against whom Ulm struggles quite a lot. Elite Warriors would flank my main force of crossbowbolt catching Heavy Infantery and Swampguards.
Abyssia would not be very different. They too rely on Heavy Infantery but they lack any missile units whatsoever. Blessed Water9 Serpent dancers would tie them up really nicely and Runners would be able to run past his main force to engage his rather whimsical casters. I would go for other summons than hordes of lower tier undead. Abyssian priests can't hit the sky lying down so a few powerful undead can engage the enemy pretty much unmolested.
That's just general strategy. If you have some more specific situation feel free to post it here.
