For starters give Gardevoir moonblast and Trace ability. Get rid pf expert belt and replace it by either a lo or a choice item. Also ,healing wish gimmicks are good too so i'd suggest running it over shadow ball.
Besides, it's never a good idea to run a mixed crobat so stick with one side of the spectrum (i'd suggest going with the physical branch and running roost and poison fang(if you want to have a pseudo toxic) or taunt for the support role. Sludge or Helmet to comply with the support role.
You rely too much on luck to win. Replace quick claw on xatu by a specs , on dragonite by a wp and on roserade by a black sludge