To my eyes, Pokémon is just a gambling game anyway, so I really don't mind adding more luck to it and taking skill away from it.
Anyway, here is a great Mega Evolution to use in the Übers metagame as far as I can tell so far:
Retribution (Scizor) @ Scizornite
Ability: Technician
EVs: 252 HP / 252 Atk / 4 SDef
Adamant Nature
- Bullet Punch
- Roost
- Superpower
- U-turn
Aside from being a great Xerneas check, Mega Scizor can also deal with any Extreme Killer Arceus very well thanks to its enormous 150 and 140 Attack and Defense stats respectively, its type combination which allows it to tank Arceus-Normal's boosted attacks very well, as well as its ability to hit the Alpha Pokémon hard with Superpower while healing with Roost.
Here is a replay demonstrating exactly that. Though do note that if you're using Mega Scizor in your team, using Arceus-Rock becomes almost a necessity, due to how unbelievably threatening Ho-Oh is in this metagame with the Defog buff, as with Stealth Rock being unbelievably easy to keep off the field, the opponent can just use a team that aims to keep on stalling while waiting for the perfect opportunity to safely send in their Life Orb or Choice Band Ho-Oh, at which point one is almost guaranteed to have one of their Pokémon at least badly-damaged unless their team contains the one true counter to Ho-Oh, and due to the fact that Ho-Oh can just switch out and Regenerate health, it can't be revenge-killed or worn down by Toxic or repeated attacks either. As a result of this, if you're not using Arceus-Rock, and therefore have no safe switch-in to Ho-Oh in your team, it would most definitely not be a wise decision to use a Pokémon such as Mega Scizor despite its ability to check what I believe to be the two top sweepers in the Generation VI Übers metagame, as giving Ho-Oh a free switch-in opportunity/Substitute is extremely detrimental.
Anyway, some good Xerneas checks and counters include the Scizor I posted above, the Lugia moveset I posted
here, Ditto, and Thunder Wave Thundurus-I, Chansey and Blissey. Ho-Oh, Ferrothorn, Jirachi and Mega Gengar also work depending on Xerneas's coverage moves.