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Crossbows (arbalests) may save your day, although hard to mass for Ulm. If your pretender can get to Fire arrows, this plus as much as possible indie archers can make much damage.
Pikes can hold them for a while .. yet will lack punch. Only Guardians and Maul infantry can put some damage in. Maybe a mix of 2 pikes+1 Guardian per square to oppose each Jotun can work.

I can't see how any of Ulm magic can help, Smiths are weak mages and Jotuns laugh when they see Blade Winds...