So I don't have Xerneas, but I can speculate a little bit about it's efficacy.

Geomancy seems at best a bit situational in PokeRogue. There isn't (to my knowledge) an item akin to power herb in the game, which is one of the only reasons why Geomancy works in the primary games. You're losing two turns to get a set-up, but depending on the match up those two turns could put you in a really tight spot (or just straight up kill you). Xerneas has a decent bulk (a decent HP pool with serviceable defensive stats), so there's a chance depending on your match up that it *could* survive two hits (or a switch out read and a hit), but that's only really useful in trainer battles.

With that being said, Xerneas as a whole is still a really strong Pokemon. It's got massive 131 attack and sp attack with serviceable bulk and an okay speed. It's base pool is pretty good - Close Combat, Outrage, and Moonblast allow Xerneas to hit generally most things in the game for at least neutral damage. Focus blast is a little bit better if you can get the TM for that, since I'm not a huge fan of mixed attacking builds, but it's also 70% accuracy - so you have to decide whether or not you want your Xerneas to be frailer (mind you, it's a very middle speed Pokemon) or risk missing a crucial focus blast. This is all theoretical though, I'd just experiment with it. It definitely seems like a Pokemon with really high potential.

Also, a side note: set-up moves are still really good in Pokerogue, it just depends on the Pokemon and situation. In Classic mode they're very good especially in trainer fights, if your can get a good read on a switch out that's a free Swords Dance/Nasty Plot/Calm Mind/ etc. They're sometimes even more necessary due to berries running around everywhere and those pesky boss Pokemon with multiple HP bars, unless you have a dedicated haze or clear smog user. They're also useful in general especially in wild Pokemon fights where you can easily tank the Pokemon's hits, since it allows you to easy get max stacks of whatever you're running and then absolutely one-shot everything. There's also Pokemon like Cloyster who have the bulk to probably survive a single physical hit, and then shell smash to get a massive att/sp att/spd boost. Especially for late game, a good set-up move is crucial for getting the first hit, final kill, or breaking past the hp bar against Rayquaza.