You could opt to go Brave but I think the idea for Mixed 3-attacks is to mostly surprise physical walls on the switch-in with a hard hitting Shadow Ball, which is why Quiet is favored.
The mixed 3 attacks set usually has 2 physical moves (Shadow Sneak, Sacred Sword are the most common I think) anyway, so I think you can play around with the EV adjustments and just shift more EVs into SpA to make up for the nature.
The mixed 3 attacks set usually has 2 physical moves (Shadow Sneak, Sacred Sword are the most common I think) anyway, so I think you can play around with the EV adjustments and just shift more EVs into SpA to make up for the nature.