We do not have official information about how the spawn works, but by observation that I could do in the test server and in lasts 3 days, the spawn of the creatures are random (when you kill a creature, it can spawn a Usurper Archer, Usurper Knight or Usurper Warlock ), like the spawn of the Glooth Bandits and Glooth Brigands .
So, if you need take Broken Longbows , the best tactic is kill all Usurpers.