The video made it look like the last phase was just an endurance test. He seems to bind you in place, then start bombarding the area with attacks. My guess would be that the play at that point is to completely stop trying to move and just burn him down. Once he gets you in place and the first missile goes off id probly try to hit manaward(or possibly just before that first missile? not having done the fight, im guessing at which timing would work best), then go full speed on the attacks. Once ward wears off, try surecast. By the time thats over the fight should be more or less done.

SannaR - I didnt mention Scathe because that can be a new player trap sometimes too. Ive seen some people think they were doing better damage because the spells were hitting faster and/or being frustrated with casting times and defaulting to scathe because it doesnt have one. Scathe damage... it feels wrong to call it damage its so much worse then true BLM potential, it doesnt refresh your AF/UI timers which can cause actual issues if you arent cognizent of it, and casting times are the class so someone too easily frustrated by that would be better off switching to RDM or SMN rather then playing BLM wrong. But all my issues with the spell aside(which i mention not for your sake, but any potential newer player reading this because they have similar issues), you are of course correct. And especially the potion bit. It shouldnt be needed since this duty includes a regen buff, but having a backup plan isnt a bad thing.