Like I said in my post, Z-geo and Psyshock/Psychic Geo beat Poisonceus.While I do agree that Xerneas is underrated rn, I never seen a Xern beat poisonceus at reasonable health unless some para full para or crit Psychic bs. Tho Arceus Poison is threatend nowadays by DualDance Dusk-Mane if no will o wisp but even if, staying in could be risky due to CM Ultra or a plain Photon Geyser from SD Ultra, I still think its close to Magearna level at checking Xerneas. Regarding (specially defensive) Dusk-Mane as a check its is still a mayor problem to Xern especially with Solgalium-Z it can drop Xern or prevent Goth from trapping. I think the biggest reason Xern is good rn is bc since Dusk Mane noone really expects much Xerneas anymore so teams that dont carry a SpDef dusk mane often times are not really prepared for it.
0 Atk Arceus-Poison Acid Downpour (160 BP) vs. +1 248 HP / 252+ Def Xerneas: 180-212 (39.5 - 46.5%) -- guaranteed 3HKO
0 Atk Arceus-Poison Poison Jab vs. +1 248 HP / 252+ Def Xerneas: 90-108 (19.7 - 23.7%) -- possible 5HKO
It actually cant break through, unless you have Perish Song or Roar before they Ingrain, but most Poisonceus have Rocks.