It sounds like connection/lag. Are you PS or PC? I only ask, as my friend is a tank on PS3, and when we did Brayflox he could not dodge that breath for sh1t. I was his healer and he used so much of my mana as almost 90% of time I had to Esuna and spam CureII to save him. I was Ether crazy! :P He found it really difficult to evade it.
His is definately lag. He is a very remote person with wireless BB to his house; so its not great.

I think you can run through the dragon tho (to his bum)-might be quicker to avoid? Or maybe you dont see the aoe right away as you are squeezed against the wall, tanking him? (assume you use the wall-tank senario). That was my friends problem-he didnt see the aoe half the time. In the end, we did a confcall, and when it appeared I shouted him to move. It was better...he still got hit sometimes

As soon as the cast starts, move. Even if you cant see the aoe. It will benifit your healer also, as with more mana thay can keep everyone alive and look after whoever is the "chosen one" (the one to get spat on-not always the tank!)

Good Luck!