The easiest boss in Elden Ring
If we're being honest, this does highlight a problem with Mohg's boss design. Contrary to what the title would have you believe, Mohg is an incredibly tough boss - if he ever gets the chance to do anything. But you can effectively circumvent the challenge of this boss entirely just by items and spells that are easily available.
By equipping the Graven-mass Talisman and Azur's Glintstone Crown, as well as casting Terra Magica, we can vastly buff the damage of our chosen spell, Comet Azur. Additionally, equipping the Primal Glintstone Blade and using the Cerulean Hidden Tear in our Flask of Wondrous Physick, we are able to manage our FP consumption to a degree that we can blast Comet Azur long enough to deplete Mohg's NG+ HP.
But damage buffs and FP management aren't the reason that this strategy works. Comet Azur is a spell that takes a long time to cast, and locks you into one position while casting. A more aggressive boss wouldn't have let us cast this spell even a quarter as long as we did. No, the real reason this strategy works is due to Mohg's actual gameplay.
Mohg's Shackle is the crucial component to this strategy. It's an item that allows us to temporarily stop Mohg from doing anything at all, for just a moment. Now, because Mohg starts the battle by slowly walking up to us, we have all the time we need - we can cast Terra Magica, drink the Flask of Wondrous Physic, and then use Mohg's Shackle to give us time to charge up Comet Azur and start blasting away.
After a moment passes, Mohg gets up, but it's already too late for him. We've already taken enough of his HP to force him into his "Tres" animation - which conveniently takes up so much time that we can take his HP down to the "Duo" animation - conveniently, this also takes up enough time that we can take his HP down to the "Unnus" animation - and before you know it, his HP is nihil.
The problem with Mohg is that he has to do these things that take up so much time. He has to begin the battle with a slow walk, allowing us time to prep. He has to fall when Mohg's Shackle is used, allowing us to blast our spell. He has to play the animations when his HP gets to certain points, keeping him from attacking us at all.
Mohg is too preoccupied with looking cool to actually be a threat to a sorcery build.