This is a very handy Baton pass set which has helped me out a lot of times. You can choose from the physical or the special one.
Physical:
Mew @ Maranga Berry
Ability: Synchronize
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Flame Charge
- Roost
- Bulk Up
- Baton Pass
Special:
Mew @ Kee Berry
Ability: Synchronize
EVs: 252 HP / 4 Atk / 252 Def
Impish Nature
- Flame Charge
- Roost
- Calm Mind
- Baton Pass
So here's the logic:
For the physical set you will pass down Atk and Def boosts from Bulk Up, an SpD boost from the Maranga berry and Spe boosts from Flame Charge. A Careful nature because you don't need SpA but you do need the SpD boost. You should also put 252 EVs in SpD since you can already boost Def with Bulk Up. Roost is there to heal.
For the special set you will pass down SpA and SpD boosts from Calm Mind, a Def boost from the Kee berry and Spe boosts from Flame Charge. An Impish nature because you don't need SpA but you do need the Def boost. You should also put 252 EVs in Def since you can already boost SpD with Calm Mind. Roost is there to heal.
Furthermore, you should put 252 EVs on HP for maximum endurance and the 4 Atk Evs are just there for Flame Charge, although it really doesn't make a big difference.