Prometheas

Li started off by discarding his human form. He had to do it. This fight was going to be serious, and though he was not willing to go entirely all out just yet, taking off his human form would let him at the least access the full breadth of his stats without worry. 

Because Lira, as a High Dragon, would have boss tier stats on top of a special ability that could potentially be equivalent to an Ultima-class spell.

She also wielded a divine-class item, making her far more a threat than any he had faced before on this world. Even more so perhaps than the Darkbeast Hand.

Fitting, Li thought, for someone whose name was known for being the greatest adventurer. 

As his human visage crumbled away in a shower of disintegrating particles, Lira leered at him. "You cast away now your disguise. Your true nature shows through. In the dread of your true form, through the skull within whose sockets I see nothing but darkness.