If I understand you correctly you want to write some sort of encounter, such as a boss spawns adds when it gets to x% of health and other things like that.
If so, that's not my area of expertise, so I recommend you either look at existing raid encounters (look at the .pl files in the zone
a particular raid mob spawns in), or post more details of what you want to do and wait for one of the gurus in that area to come along
