How Should I Build MegaRay?

[Marcusube]

Rayquaza @ Life Orb
Ability: Air Lock
EVs: 100 Atk / 156 SpA / 252 Spe
Naive Nature
- Dragon Ascent
- Draco Meteor
- Fire Blast
- Protect

This set is pretty basic for the format. Dragon Ascent is Mega Rayquaza's strongest STAB physical attack. Draco Meteor is its strongest special STAB. Fire Blast hits the steel types that resist the first two attacks.

156 SpA Life Orb Mega Rayquaza Draco Meteor vs. 252 HP / 0 SpD Primal Groudon: 191-226 (92.2 - 109.1%) -- 56.3% chance to OHKO
100 Atk Life Orb Mega Rayquaza Dragon Ascent vs. 0 HP / 0 Def Xerneas: 164-192 (81.5 - 95.5%) -- guaranteed 2HKO
100 Atk Life Orb Mega Rayquaza Dragon Ascent vs. 0 HP / 0 Def Primal Kyogre: 172-203 (98.2 - 116%) -- 87.5% chance to OHKO
156 SpA Life Orb Mega Rayquaza Draco Meteor vs. 4 HP / 0 SpD Mega Kangaskhan: 175-207 (96.6 - 114.3%) -- 81.3% chance to OHKO

The thing is, the only way to EV Rayquaza is against common threats in the meta and even then it doesn't OHKO them with the standard LO set. Its going to be hard to get a free turn to Swords Dance without getting attacked and even then, if your sash gets broken, a strong priority move will KO you

Another way to run Rayquaza is a Dragon Dance set with either Leftovers, Yache Berry, Haban Berry, Lum Berry or Roseli Berry. Something like this:

Rayquaza @ Leftovers
Ability: Air Lock
- Dragon Dance
- Protect
- Dragon Ascent
- Earthquake / Extreme Speed