Gardevoir counters
Gardevoir isn't very bulky, but she really hits like a truck, even against neutral attackers.
Mewtwo with PC/SB obviously. But that's why I have a C/SB Gardevoir defender to troll Mewtwo users, besides my C/DG one. C/SB will also hit Metagross hard.
Snorlax with Lick/Heavy Slam might be a good tank counter.
Answers
Raikou is a great choice- Gardevoir has no bulk so even neutral damage takes it down fast. Fire types for the DG resist. The water types like vaporeon, gyarados and kyogre will also eat all Gardevoir attacks for breakfast and take it down easily. 2x SE charge beam doesn't matter since its still worse than stab confusion.
Yeah, pretty much aything neutral will be good enough. Still the main use is forcing a switch out from Machamp.
I assume it will have Dazzling Gleam and use Moltres with Overheat against full motivation, or Flareon with Flamethrower against low motivation. When I eventually evolve my Aggron, I might start using that.