Title. The champions must have at least a 1/10 chance of beating him. That means That they won't necesarly win in most cases, but a 1/10 is enough for them to be included. Any condition for the 9champions to be able to beat Xerath is allowed (ex: prep time, full Knowledge of his abilities) . Rules: It must be a 1v1. No amplifications like Ascension if the champion doesn't have anything to do with It. In-lore.
Ryze, Syndra, Veigar, Vel'Koz, and Lissandra.
Maybe LeBlanc, if she finds any of his weaknesses.
No idea HOW he would beat him, but im SURE that Xerath would not kill him... so, there is a chance.
... Same goes for 
and maaaaybe 
(but that is even more farfetched). Skaarl could also fit here, but she is a coward.
And we could also include undeads in the list like 
since they can damage the "soul" of Xerath.... and even 
could kill him since he kills everything (in theory). 
Also could fit here, since she can steal his soul or something like that.
Since we are including 1/10 chance of beating him, i would say that 
could defeat him as well, since she can turn him into a little animal or flower and then beat him.
Also, since he is an Asceded... 
could trick him like she did with the other Ascended (Darkins) and defeat him. I guess that 
could do something similar with the power of the moon?
With prep time and full Knowledge of his abilities, i think that scientists like 
and 
could defeat him as well.... since "full Knowledge of his abilities" would mean that THEY would know more about his abilities than Xerath himself and could counter him with the power of SCIENCE. I mean, Viktor can create electric storms with his current knowledge, imagine what he could do if he had the arcane knowledge of how the skills of Xerath work. Heck, they could even create their own Ascention Ritual in theory.
EDIT: I forgot 
... but that is obvious.
Outside of Lore: so far riot hasn’t showed us the power and friendship thing so much except between xerath and azir.
Now to context who knows maybe azir can after all Azir’s rage is widely known. The second thing is azir was ascended from a “chosen”respective after saving sivir. IMO it seems Azir him sled could shift Xerath.
The targonians also learned to reverse ascension.
The next one goes to ryze, Who know if Xerath is after a rune stone and ryze gets in a battle vs him.
And last but not least the most unexpected pick, the ruined king. Not only did he made a land of peace and power deadly and undead but he fused the very essence of men and animals in his rage. This shows he could give xerath a death sentence.
An honorable mention is zillean. For the powers of times
Another is the god of illaoi. The powers this entity holds are very quite but we also do not know is full power
When he fought Xerath with Renekton, he had no time to prepare or to understand his enemy. I believe Nasus would now have a chance to defeat Xerath since his mind is focused on that. Definitely not as powerful as Xerath, but master strategist and all that.
He comes out of his forge to put other demigods back in their place, so he's considerably powerful. It's possible he'd be able to defeat Xerath or forge an item able to go against his magic.
We don't know if Xerath's magic would be too much for Galio or if it would actually give him what he wants: enough energy to stay awake. It's possible he's strong enough to defeat Xerath.
Wow. According to this thread's messages, everyone beats Xerath. Even Viktor and Jayce...
It's funny how much underestimated he is. Nasus, Warrior King of the Ascended, an high tier god-warrior, couldn't even last minutes... And I promise you he is far durable than a mere mortal like Jayce.
I would assume ekko could try enough times to eventually have a chance.
Assuming he always had the drop and was fast enough to rewind before he died.
Also malzahar may be able to defeat him by trapping him in his fears.
would flat out disintegrate the Sarcophagus leaving Xerath unstable.
Would also be able to break Xerath's confines but unlike most would more than likely also be able to survive the resulting explosion
Aspects nuff said
Ancient demigods nuff said
Given prep time I'm certain Vel'koz would find and exploit a weakness
Eats magic for breakfast
Cho'gath seeks to consume all life and void born have limitless potential
Rhaast would certainly prove a challenge for Xerath
Since kindred is death n all.
falls into the 1/10 chance category and if he HAD to he could use the world runes.
Given enough time Kha'zix could just evolve even further and further
Some obvious ones too 
Celestial nuff said
Stated by riot that under the right conditions he could beat xerath
Scales to aatrox
Viktor vs. Xerath could be... Explosive. Hitting someone that has commonly been refereed to as a barely stable nuke with something called a Chaos Beam or Chaos Storm are not activities I would recommend to anyone looking to live for very long. An actual conflict could end very similarly to the doom of Shurima.
