Look what I do is select a scene where the boss will appear so create the logic there.
First set up all sprites and objects then assign the scene to an event sheet in my case is scene Level_10 and Level10 Events
I give you some tips on how to set up this
All my events are separated in Groups so is easy to find and debug errors
Then inside each group the events logic like
When the boss must start to move.
When the boss must run or fly
Hope this helps