Turns out that Fia needs a bit more than just to hold you. You'll need to get the Carian Inverted Statue and Mending Rune of the Death Prince for her, too.
The God-Devouring Serpent is a gimmick boss in Elden Ring, and it gets even more interesting once you reach the second form: Rykard, Lord of Blasphemy.
Mohg is a boss you might fight twice in Elden Ring, depending on how your playthrough goes. The Lord of Blood version, however, is the hardest and most involved.