Jirachi @ Leftovers
Ability: Serene Grace
EVs: 248 HP / 240 Def / 20 Spe
Jolly Nature
- Wish
- Protect
- Thunder Wave
- Iron Head
M-Gardevoir Jirachi core is, in my opinion, one of the bulkiest. Jirachi gives to M-Gardevioir the physical bulk, the poison immunity and the steel resistance it needs.
As Wisher, Jirachi may help teammates and It's enough bulky to resist to HyperSpace Fury Scarf Hoopa-U 252 Atk Hoopa Unbound Hyperspace Fury vs. 248 HP / 240 Def Jirachi: 306-360 (75.9 - 89.3%) -- guaranteed 2HKO.
Jirachi is still useful against M-Pinsir (Unless this last use EQ instead Close Combat).
But this core is weak to Fire Fast sweeper like Chandelure, Entei,... Gengar and Excadrill (if not banded under Sandstorm).