[Sheazy]

When I did this event a few patches ago, it was pretty buggy. That being said, the summoners are not on a spawn timer. You spawn them by going and killing that ghost guy who's in a building towards the back-right of the area. I don't remember his name, sorry :P But once he's dead, the summoners will spawn at one of the places around the area.

Also, you don't kill the summoners, just the gargoyles. I believe they come in 4 waves of 2. But like I said, I've attempted this event a number of times and only successfully completed it twice. Usually the gargoyles just stop spawning or something else weird happens.