Gengar @ Gengarite
Ability: Illusion/Prankster
EVs: 80 HP / 252 Def / 176 Spe
Timid Nature
IVs: 0 Atk
- Encore
- Perish Song
- Ice Beam
- Flash Cannon

Has enough speed to outspeed Arceus after Mega, and still kills 252 hp Mega Ray and MDiancie without SpA investment. Alternatively you can run 136 Spe to outspeed 115 after mega.
252+ Atk Sky Plate Aerilate Mega Rayquaza Extreme Speed vs. 80 HP / 252 Def Mega Gengar: 262-309 (93.2 - 109.9%) -- 56.3% chance to OHKO
252+ Atk Sky Plate Aerilate Mega Rayquaza Extreme Speed vs. 120 HP / 252 Def Mega Gengar: 262-309 (90 - 106.1%) -- 37.5% chance to OHKO
Full attack or LO Aerilate isn't too common high ladder