Stirok is actually spawning in randomly and despawning even when the event hasn't bee triggered. I've been haning out in The Rocketyards trying to kill him for the quest and I kept hearing a roar and catching glimpses of something big despawning. Well, I was right next to him the next time. He spawned in and despawned shortly after. I was able to catch him after that and kill him two or three times before he stopped respawning. I can upload a video if need be. Also, none of the kills counted for the quest.