It looks like he split the siege engines and demolishers up with one of each in opposing corners. 66% chance at each switch to target the opposite corner. I couldn't see the choppers in the video, but you'd probably want them near the middle to drop the tar to slow down FL. I'm curious what his setup is like to get targetting to work. You'd obviously need all 10 windows visible to see if your shots are hitting the boss and quickly adjust when you see you're missing.

The main reason it feels clumsy when you do it is most people have no idea what they're supposed to do. People don't shoot down pyrite, demo's don't roll their stacks, choppers try to dps with sonic horn, etc. You can 4man him with all the towers down pretty easily with just the 2 demolishers.