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I have found the way to overcome M-Rayquaza
>Calm Rayquaza
>NO mega
>make sure it has 0 IVs in attack
>Expert Belt
>252 Sp Atk Rayquaza Foul Play vs. 252 HP/252 Def Mega Rayquaza: 107.7-112.4% (100% chance to OHKO)
You will always attack last, thus giving you a chance to scout Rayquaza and hit last.