Probably dying to the Fire+Ice mechanic in the final phase that will instantly kill you as soon as it starts since there is no one to pass the debuff to. Just gotta kill Nael before that part starts, which is not easy solo. I can kill him instantly at the start of the fight with max level Blue Mage if I get DH Crit Sting. Just gotta do a little bit of damage first, but not enough to skip to golems, then boom, and if you're lucky it kills him right there.