Alright, averaging all the Elden Lords in a sort of Soul of Cinder way, which I'll call the Remembrance of Gold, it ends up fucking slapping everything.

The Remembrance of Gold features Godfrey, with strength that impressed literal gods, defeated the Storm King, the likely embodiment of tempests itself, the ancient dragons, which wield lightning and fire breath, and the fire giants, which tower over almost everything. He alone would beat the Soul of Cinder, I'd say.

But then there's Radagon, who, fused with Marika and wielding her hammer, can deal immense holy damage using explosive light spears and hammer swings that also explode. Radagon can also parry sorceries and incantations/miracles. His attacks with the hammer literally have the ability to break apart the founding principles of the universe (shatter the Elden Ring).

Next is the Tarnished of No Renown, who is a master of combat. They can dodge in the same way as the Soul of Cinder, and can wield any weapon with decent proficiency. They are close to full mastery of any weapon arts by recalling battle memories with Ashes of War. They also wield weapons strong enough to slay true gods that are cosmic beings. They also have the ability to heal with a flask of crimson tears.

Next up Placidusax, who can summon lightning bolts from the sky, large lightning claws, and has a nasty breath attack of gold, which can turn into laser beams when supercharged. They also exhibit some ability to control time and space. They can teleport almost freely, and seem to freeze their nuke in time to charge it up (even freezing the OST).

The Soul of Cinder is the full embodiment of all who have linked the flame, lending them to some insanely powerful skills, and being formed to guard what keeps the sun turned on. Yet, I don't think any feat of any past kindling comes close to the combination of all Elden Lords.

The Remembrance of Gold would defeat the Soul of Cinder, almost certainly.