Man, I don't think killing before gnats is even possible with a MNK in the party since you can't hit it from behind. Group I run with most frequently is WAR/MNK(me)/SMN/SCH and we can clear on the repel following the gnats sometimes, but never faster than that, at least not yet. Getting a Garuda book for our SMN will help (I got my claws in 1.x), no doubt, so maybe at that point. We mess up the last boss pretty quickly usually before or during the 3rd catastrophe though (I can hit that boss from behind which helps the DPS a lot).
Tank runs through dragon on pull, then back around to the right and positions boss like this. Everyone else can just hang out on his side. The red line indicates the direction that the statue will block the Catastrophe attack. No one has to move an inch during Catastrophe, not even the tank (so no one gets hit by breath but the tank). When gargoyles spawn, 1 DPS (ranged preferred) grabs it and pulls it towards the entrance so it doesn't drop an AoE on the healer or tank, then other DPS helps finish it off. Healer only needs to move if you get pinklined.