You need to own/build your OWN units (extra units - dwarves, vikings, Nilfgaardins.. are not required). The description would lead you to believe you need all NON-Gascon and NON-Reynard units, which to my understanding would be all units on the "middle path" in both Workshop and Training Grounds, that also do NOT appear on either the left or right paths of the three with Unit + names...

However, I DID get the throphy by building a Wagenburg +, which is on the "Reynard branch" in Workshop. Still, since the NORMAL Wagenburg is available to build by default - I assume all starting units are "non-Gascon, non-Reynard", even if their + version are alligned with one of the lieutenants.. Go figure.

Hope that helps.