Diancie-Mega @ Diancite  
Ability: Clear Body  
EVs: 224 HP / 52 Def / 116 SpA / 116 Spe  
Rash Nature  
- Rock Tomb  
- Moonblast  
- Diamond Storm  
- Skill Swap

Matchups:
Dnite: Mini tomb -> Mega moonblast 
Gyara: Mega tomb -> Moonblast, if you think its flyz and moonblast isnt killing you use Diamond storm second turn 
Zard: Mini tomb -> Mega rock moves
Zera: Mini tomb -> Mega skill swap, or just raw skillswap in mini form. Doesnt Rly matter (we do lose to band itail)
Garde: Mini tomb -> Mega double attack if they beam t1, skill swap if they cm t1