So, they patched the game awhile ago so that you could reset garrisons. If you have completed the game, but have not killed 25 blood dragons. No worries, and no sitting around waiting for them to spawn.
Press Start -> Gameplay - > Reset Garrisons
Once the garrisons are reset and red and repopulatedwith omega soldiers, like at the beginning of the game. You get the killstar and can demolish anything that stands in your way, make sure to deactivate the shield which will spawn a blood dragon per garrison to kill.