Crobat is useless. Neither useful is Toxapex, due to the presence of Arceus Poison in Nat Dex AG.
Here is what Chloe, a expert AG player wrote:
Toxapex can run Wacan Berry to stop Max Lightning and then Haze. Thankfully, Toxapex does have other uses and hence can do other things such as pivot into attacks later or set Toxic Spikes. This still doesn't justify using it though. If you really need a Xerneas, Eternatus, Ho-Oh and Marshadow check then you could potentially justify this, otherwise I'd find something else.
Here's the best Poison-type for Xerneas:
Arceus-Poison can Dynamax itself and take on Xerneas; however, is very inferior to Necrozma-Dusk-Mane, and is virtually useless against any other Pokemon. Max Ooze is also fairly weaker than other Max Moves due to Fighting- and Poison-type moves getting lower boosts. It can also run Psych Up to boost to the same level as Xerneas, stall out Dynamax with Recover, and then do good damage to an opposing team but this is very rarely worth it. Arceus-Poison will be dead-weight against most teams lacking Xerneas.
You can also try Amoonguss, though I don't justify using it for checking only Xerneas.
Assault Vest Amoonguss is a very very solid check for Xerneas, and a soft check for other select Pokemon such as Primal Kyogre and support Arceus, but that is about it. It stops all of Dynamaxed Xerneas's attacks, taking at the very most 95% from the subpar Max Mindstorm (Psychic or Psyshock). It can Clear Smog away Xerneas's boosts and then pivot to a supporting partner such as Ho-Oh to regain health and be usable later. There are issues with this however. If a Xerneas has Psyshock and opts not to Dynamax, it can OHKO cleanly. Thankfully, Psyshock is considered a bad choice for coverage in a metagame where Necrozma-Dusk-Mane and Ho-Oh are on virtually every team.
Why use Clear Smog to take away the boosts when you have Heart Swap Magearna? Also, Ditto can check a weakened Dynamax Xerneas, but it depends. The best counter for Xerneas is Dusk Mane Necrozma indeed.
While there are not many viable options to check Xerneas, Necrozma-Dusk-Mane stands out as one that is capable of being fit onto virtually any team. Xerneas very often has perfect coverage in order to OHKO the entire metagame whilst in a Dynamaxed state; Necrozma-Dusk-Mane is an exception to this. Unfortunately, with Xerneas's doubled HP, there is room for error.
Necrozma-Dusk-Mane doesn't take Max Flares and Max Lightnings that well in its base forme, meaning it gets at least 2HKOd by either of those, regardless of what set it decides to run. This is alleviated by using your own Dynamax on Necrozma-Dusk-Mane. Both offensive and defensive variants are able to beat Dynamax Xerneas post-Geomancy by Dynamaxing themselves. For example:
+2 252+ SpA Xerneas Max Flare vs. 252 HP / 0 SpD Prism Armor Dynamax Necrozma-Dusk-Mane: 369-435 (46.4 - 54.6%)
By clicking Max Guard on your first dynamax turn, followed by Max Steelspike, followed by Max Guard you are successfully able to outstall a Xerneas's dynamax. When facing a competent player, be weary that Xerneas may opt not to Dynamax right after Geomancy in order to waste a turn of Necrozma-Dusk-Mane's Dynamax. This means that even with Dynamax, Necrozma-Dusk-Mane is not a guaranteed check to Xerneas.
Necrozma-Dusk-Mane can run 252 HP / 252 SpD with a SpD boosting nature in order to handle this scenario better, or even run an Occa Berry; however, it should be stated that these options are very very situational and will not be necessary often at all. If the Xerneas player incorrectly predicts the Max Guard, it can take substantial damage from Max Steelspike and fall regardless.
Hope this helps!