I've been playing the metagame more now and I'm just not finding Xerneas all that overwhelming, whether it be because my team checks it well or because I'm not finding myself able to use it in matches as much as I'd like. Stuff like Primal Groudon and Ferrothorn wall it to hell and back, and Fairy coverage doesn't seem to be all that useful this early in the metagame. Geomancy is still ridiculously hard to pull off, and the things you'd want to be hitting with Fairy coverage (like Mega Rayquaza and Mega Salamance) will easily chunk you, if not outright KO you, before you get the Geomancy off.
On paper, it sounds like a huge threat. In game, however, it doesn't seem to deliver much, in my opinion anyway. Maybe set up mons aren't my type of play style...
Now here's the thing. Even when you consider these things, you have to keep 1 or 2 things in mind:
First of all, every Pokemon (well, besides Kangaskhan and Primal Groudon) loses to something. Xerneas is no exception as it heavily relies on its fairy coverage and in Geomancy to attack, but let's be real. Every team is FORCED to run a stop for Xerneas, and a lot of teams actually run 2 because 1 isn't enough. That kind of pressure since teambuilding stage makes Xerneas already A or A+ material by itself.
Now, let's think about how does one think about stops for Xerneas. Something that prevents it from freely setting up Geomancy, right? Ok so what stops for Xerneas do we have to stop it AFTER the Geomancy? Not much besides like Ferrothorn, really. This forces the player to try and do its best to alright prevent it to set up a Geomancy by the means of Fake Out / Taunt but then again, Xerneas can capitalize on it and attack / Protect freely while you look like an idiot and you let the partner do everything.
Also, Xerneas is part of one of the most deadly if not the deadliest combinations in the game, this being Xerneas + Smeargle.
I'm pretty sure Xerneas is a metagame defining Pokemon which justifies its placing at S rank:
Reserved for the top threats in the VGC16 metagame. The Pokemon in this tier are able to perform multiple roles to tremendous effect, whilst having few to no flaws. Support Pokemon in this tier can easily create free turns for their teammates without creating clear openings for the opponent. Any flaws that these Pokemon have can be taken care of with little to no team support.
Top Threat? Definitely.
Multiple roles? ehhh... not really. Actually in this aspect A+ fits it more, yeah.
Few to no flaws? Definitely. Has amazing bulk, good speed tier (welp, outspeed 90s which is kind of nice), a really good ability, Geomancy and it's very strong. Few flaws? yep, you're vulnerable to taunt/knock off and you can't touch steel-types but that's pretty much it.
Any flaws can be taken care of with little to no team support? Just slap a Primal Groudon together with Xerneas and you're golden. Not even kidding. Ferrothorn and Smeargle can also support it very well, so yeah...
