Xatu (M) @ Leftovers
Trait: Magic Bounce
EVs: 4 HP/ 252 Def/ 252 Sp.D
Nature: Calm (+Sp Def, -Atk)
• Calm Mind
• Confuse Ray
• Stored Power/Psychic
Calm Mind: Calm Mind is very helpful for Xatu, as it's a Special Attacker. This also helps boosts the power of Stored Power. Use it twice after Confuse Ray, and Stored Power's power would be ( 20 × 4 + 20) 100. If you're going with Psychic, then it'll also be useful.
Confuse Ray: This one is simple. Just stall the opponent out and use that opportunity to raise your stats (Calm Mind).
Stored Power/Psychic: I don't think many people have heard of Stored Power move, but it is to be feared. For each stat boost (positive), the move's power'll increase by 20. The maximum power is 860. Use it well and it'll benefit you. However, if you're worried if the opponent has Roar, then go with psychic.
Roost: This is pretty obvious. Use it when your HP is low.
P.S This is my very first post, so correct me if I'm wrong about anything ^-^