Hoopa-Unbound @ Choice Scarf  
Ability: Magician  
EVs: 252 Atk / 104 Def / 152 Spe  
Adamant Nature  
- Gunk Shot  
- Trick  
- Hyperspace Fury  
- Laser Focus  

Deoxys-Speed @ Buginium Z  
Ability: Pressure  
EVs: 248 HP / 252 SpA / 8 SpD  
Timid Nature  
IVs: 0 Atk  
- Signal Beam  
- Taunt  
- Amnesia  
- Recover  

Mew @ Aguav Berry  
Ability: Synchronize  
EVs: 252 HP / 4 Def / 252 Spe  
Jolly Nature  
- Iron Defense  
- Taunt  
- Rock Tomb  
- Soft-Boiled  

What they do:

Hoopa-Unbound is a stall breaker as well as all out attacker. 152 speed allows him to outspeed all mega-metagross and it lives a Meteor Mash from some Mega-Metagrosses.
Deoxys-Speed serves as a blanket check to special attackers as well as being able to beat Hoopa-Unbound by outspeeding and OHKOing with Z-Signal Beam
Mew serves as a blanket check to physical attackers as well as being able to beat stall deo-s. It always lives a 252+ Victini Z-V-create -> V-create as well as not losing to taunt variants due to it's speed control.
3-0s:

Metagross-Mega
Bulky Mega-Metagross with Meteor Mash, Bullet Punch, and Laser Focus can potentially 3-0.