Simple Dragon Dance set
AG Creator (Rayquaza-Mega) @ Life Orb
Ability: Delta Stream
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Extreme Speed
- Dragon Ascent
- V-create
- Dragon Dance
Dragon Ascent is needed to Mega-Evolve. Extreme Speed is priority and targets faster Pokemon. V-Create melts Ice-type along with Necrozma-Dusk Mane. Dragon Dance is set-up, and at +1, only the bulkiest Pokemon in this whole world can take its hit. Adamant is chosen over Jolly since you already outsped a lot of things after a +1 Dragon Dance. Life Orb is chosen, and for other weird choices would be Lum Berry (Lol) and other things.
BUT FOR THE FUTURE OF THIS WORLD DON'T USE SASH RAY!
Calculations
At +1 Atk, without Life Orb, see your damage input against Ho-Oh.
- +1 252+ Atk Rayquaza-Mega Dragon Ascent vs. 248 HP / 208+ Def Ho-Oh: 331-391 (79.7 - 94.2%) -- guaranteed 2HKO
Now, you shall see yourself getting a burn from Sacred Fire or Whirlwind-ed back while Ho-Oh gets that HP back from Regenerator.
AND if you held Life Orb, it surely would have been a KO.
- +1 252+ Atk Life Orb Rayquaza-Mega Dragon Ascent vs. 248 HP / 208+ Def Ho-Oh: 430-508 (103.6 - 122.4%) -- guaranteed OHKO
Now, VS opposing M-Ray.
- 252+ Atk Rayquaza-Mega Dragon Ascent vs. 0 HP / 4 Def Rayquaza-Mega: 274-324 (78 - 92.3%) -- guaranteed 2HKO
You don't get the KO without Life Orb. On the other hand;
- 252+ Atk Life Orb Rayquaza-Mega Dragon Ascent vs. 0 HP / 4 Def Rayquaza-Mega: 356-421 (101.4 - 119.9%) -- guaranteed OHKO
VS Yveltal
- 252+ Atk Rayquaza-Mega Dragon Ascent vs. 0 HP / 0 Def Yveltal: 288-339 (73.2 - 86.2%) -- guaranteed 2HKO
While if you had Life Orb;
- 252+ Atk Life Orb Rayquaza-Mega Dragon Ascent vs. 0 HP / 0 Def Yveltal: 374-441 (95.1 - 112.2%) -- 68.8% chance to OHKO
This explains why Life Orb is possibly the best item on it. Using Focus Sash on it is totally bad; it doesn't get a lot of important KOes where it would have done. As such, there arises no reason to use Focus Sash.
Thanks for reading.