It's not easy, but it can be done.
Dark Souls 3: how to one hit KO Pontiff Sulyvahn
Dark Souls 3 has several difficulty spikes and the Pontiff Sulyvahn boss fight is definitely one of them. Not everyone will struggle with this dual-wielding asshole, but those who do will bash themselves against him again and again.
My own strategy is "summon someone who knows what the heck they're doing", but YouTuber neptunusequester had a better plan, tackling the boss as a one hit knock out (1HKO) challenge.
You can see the fight in all its glory above. Although it looks like the boss is hit twice, he's not; that's just the riposte (post-parry front critical hit) animation for this weapon when utilising the Hornet Ring. The attack button was pressed once, and the Pontiff went down, legit. Absolutely tremendous.
So how was it done? Redditor HolidayForHire put it together from the visual cues, with the triumphant Unkindled chiming in to confirm and correct.
At just Soul Level 70, this build uses no armour to compensate for the heavy equip weight of the weapons involved, and includes the Hornet Ring to buff critical attacks, and the Red Tear Stone Ring, which increases attack when HP is low.
Before engaging Sulyvahn, the player achieves low HP, applies the Dragon Torso Stone for its transformation buff, switches to the Sunlight Straight Sword to apply its Oath of Sunlight weapon art buff, applies the Protection of the Deep miracle for a further buff, switches to Yhorm's Great Machete to apply its Warcry weapon art buff, and hits the Dragon Torso Stone once more to activate its very short duration buff (getting the timing right was very difficult, apparently).
The player then neatly parries Sulyvahn's first attack and switches to the Morion Blade in their left hand, adding another attack boost due to low HP. With all those stacking buffs, the attack unleashed by a fully upgraded Yhorm's Great Machete is absolutely devastating.
This build is only possible on New Game+ as some of the necessary materials aren't available until later in the game, unless you get lucky in trades. But it's amazing to see what can be done with thoughtful exploitation of the game's interlocking systems; think about that next time someone tells you Dark Souls is all about twitch reflexes rather than RPG mechanics.
The total buff achieved by all this is somewhere in the range of 200-300%, apparently, so if you can parry and riposte reliably you could take the Pontiff down in about three hits with a similar quality weapon and stats, minus all the extra buffs. It's really worth mastering that skill - but I personally probably never will (all hail Cheeseburger Assassins).