I mean for Double Battles.
Gardevoir has a great ability for doubles - Telepathy, making it take no damage at all from allies attacks. If yours has that, I highly recommend abusing that with a strong Surf or Earthquake. Another option is to pair it with a Fighting type, so if Gardevoir is getting targeted by a Dark type, it can protect and the Fighting type can hopefully KO. This also works with a type to cover Bugs, such as Fire or Rock. I'm not that sure about the viability of that strategy though, as I am not that experienced in doubles.
Hope I helped! :)