If I'm reading this right, Cray has offensive stats of 130/130/97. That makes it a slightly faster yet frailer Kyurem-N, statswise, with a stronger secondary STAB, and (much) better coverage, as well as the options for and boosting (notably, if DDance, +1 Adamant outspeeds everything up to Jolly M-Lop). It's still weak to rocks, though, but it looks an awful lot like a mildly weaker version of regular Ray in standard.
Xerneas suffers slightly from dropping down to base 89 speed - it has to run max speed timid to avoid being revenged by Excadrill in sand. It's obviously weaker but +2 base 105 with the Aura boost is still pretty scary for offense especially, it also destroys sableye for obvious reasons so pairing it with a strong physical attacker sounds good.
Speaking of strong physical attackers... Ekiller still sounds like a monster - SD/Espeed/Recover/Earthquake or Shadow Claw or Shadow Force is a very good cleaner.
Honestly the 3 work pretty well together.
Arceus @ Lum Berry / Life Orb
Ability: Multitype
EVs: 248 HP / 252 Atk / 8 SpD
Adamant Nature
- Swords Dance
- Extreme Speed
- Recover
- Earthquake / Overheat
Xerneas @ Power Herb
Ability: Fairy Aura
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Geomancy
- Moonblast
- Focus Blast
- Psyshock / Aromatherapy
Rayquaza @ Choice Specs / Expert Belt
Ability: Air Lock
EVs: 4 Atk / 252 SpA / 252 Spe
Timid/Naive Nature
- Draco Meteor
- Overheat
- Surf / *insert preferred coverage*
- Dragon Ascent
or
Rayquaza @ Life Orb
Ability: Air Lock
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant/Jolly Nature
- Dragon Dance / Swords Dance
- Dragon Ascent
- V-create / Overheat
- Extreme Speed
EVs on them are definitely not optimal, Arceus can be EV'd to live a certain hit or outspeed whatever. The idea is that all three can sweep teams once certain bulky mons are removed, such as (for Arc) Sableye and bulky steels not weak to EQ eg ferro and skarm. Xern comes in for free on Sableye (you can even run Aromatherapy to be a cleric), and the first RayRay set is to lure Skarms and Ferrothorns and destroy them with Overheat (although you can also do this with Arc if you'd prefer). The second set is more standard, and works with Arc to wear down their shared checks. An Adamant nature is ok with DDance, but you're outsped by Scarf 85s and above (aka Uber base 90s). The core seems like it'd struggle against Twave Mega Slowbro, as Xern hates paralysis without Aromatherapy and Arceus isn't breaking it easily thanks to Scald (partly why it carries Lum). Unaware Clefable could also be a royal pain, avoiding the 2HKO from Espeed (I'm not sure how it fares against the others). Bisharp seems like a decent offensive partner as it can potentially beat both of those.
Obviously there's a lot more you can do with these, Arc can literally do anything you want, and Ray is fast enough to run Scarf, or can run more of a mix depending on what you want it to beat, etc. Xern is probably stuck running Geo to have any niche, though. Otherwise just use Clefable.
also since STag is banned in OU, Gengarite should probably be banned.