There has not been a patch released yet for the North American Xbox version (not sure about other regions).
If you don't destroy the generator until after the fight, Xanlosch never summons the flying demons. If you destroy the generator after talking to him, the summoning works properly. The game doesn't expect you to destroy the generator before getting the first cutscene, so that is what causes the problem.
If you don't have an earlier save to revert back to (perhaps the last auto-save), or if that was too long ago, see Lar's post
here on how to send them an Xbox save.
