On phone so hard to go in lots of detail. Usually I'll start the ability with a persistent effect which triggers the range actor. The actual ability attack will then happen after the persistent effect is finished. You adjust the persistent effect duration to give the player time to avoid it.
If you look at my projects on this website I do this on both the infested siege tank and the planetary fortress in my Mapster's series in the Cleanup Crew map. There are also plenty of other maps with boss fight you can look at. The hots primal Zerg boss fight map is also good. I think some of those were done via trigger.