I have completed this event successfully 3(?) times or so in the last 10 days since I have come back. For what it's worth I noticed a small hiccup between phases at least one of the times. I believe the script looks something like this.
1.) Kill Guardian of Doomfire - Spawns trash leading to Fennin.
2.) Kill all trash leading to steps - Spawns 4 demons with more trash on steps.
3.) Kill 4 demons and trash - Spawns 4 mini bosses in Fennins room.
4.) Kill 4 mini bosses - Spawns Fennin and adds new elite guards to Fennins room and to the stairs.
I was completing part 2 killing all the trash leading up to the steps, I cleared the last pack before the stairs and the 4 demons didn't pop. The stair demons ended up popping with adds like 15-20 seconds later or so, When I remember them always popping almost instantly upon the death of the last trash mob. It Just seemed like an odd hiccup at the time but it may be related.
Other than that the script seems to be working just fine. And as a side note to 1BFC I will gladly help you and your crew kill Fennin. My main is Remember, send me a tell any time.