ban magic bounce. the only ways to hurt it are:
syncronise (somewhat-simple, toxic, it gets bounced back, and now they get it too, but one lum berry and that's it)
trick + AV + klutz (easy, but can be countered if holding a mega stone)
nature power (counter, but magic bouncers are bulky)
mold breaker (stopped by simple beam xatu)
skill swap (stopped if you switch)
...good luck.
Xatu @ Heracronite
Ability: Magic Bounce
EVs: 252 SDef / 252 Spd / 4 Def
Calm Nature
- Refresh
- Calm Mind/Light Screen
- Simple Beam
- Toxic
heal bell/refresh to get rid of syncronise + toxic, calm mind/light screen to get special bulk ++ for nature power, Heracronite to stop trick + assault vest, simple beam to kill mold breaker, toxic to... deal damage.