Just make sure you don't break the leg chain with the Rotten Breath (it's pretty easy), and if you're not sure you got much damage to the chain, just hit it once or twice with the Reduvia. The right leg hits are just to build up a bit of bleed, but you can use the weapon art up to 4 times on the left leg and still have enough FP for a dragonmaw. If you get 2 dragonmaw hits to the weakspot and a bleed, and break the chain around 8k, first phase is guaranteed to end before you have to deal with any fire shenanigans.
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