Actually uh... Taking Tornadus-T instead lol.
Tornadus-Therian @ Assault Vest
Ability: Regenerator
EVs: 96 HP / 160 SpA / 252 Spe
Hasty Nature
- Hurricane
- Superpower
- Knock Off
- U-turn
Tornadus-T is a decent check to Mega Alakazam when equipped with an Assault Vest, as Mega Alakazam fails to 3HKO with Psychic, while in turn Tornadus-T 2HKOes with Hurricane or Knock Off. If Stealth Rock is up, Mega Alakazam can 2HKO, but Tornadus-T can pivot out with U-turn to come in at a later time to finish Mega Alakazam off.
Calcs:
252 SpA Mega Alakazam Psychic vs. 96 HP / 0 SpD Assault Vest Tornadus-T: 135-159 (41.7 - 49.2%) -- guaranteed 3HKO
0 Atk Tornadus-T Knock Off vs. 0 HP / 4 Def Mega Alakazam: 134-158 (53.3 - 62.9%) -- guaranteed 2HKO
0 Atk Tornadus-T U-turn vs. 0 HP / 4 Def Mega Alakazam: 144-170 (57.3 - 67.7%) -- guaranteed 2HKO
160 SpA Tornadus-T Hurricane vs. 0 HP / 0 SpD Mega Alakazam: 156-184 (62.1 - 73.3%) -- guaranteed 2HKO