Here's my Mega Gardevoir setup:
Nereid (Vaporeon) (M) @ Leftovers
Ability: Water Absorb
EVs: 244 HP / 136 Def / 128 Spd
Jolly Nature
- Scald
- Baton Pass
- Acid Armor
- Substitute
Xana (Gardevoir) (F) @ Gardevoirite
Ability: Trace
EVs: 4 HP / 252 SAtk / 252 Spd
Timid Nature
- Calm Mind
- Hyper Voice
- Shadow Ball
- Focus Blast
I send out Vaporeon vs. a weak type or starting hazard setter and sub. If they switch, I'm able to set up acid armour. If they decide to set up, I either boost with another acid armour and then baton pass, or I just baton pass immediately to Gardevoir. Behind the sub, I use calm mind and Mega-evolve. They will often then take down the sub, but then I'm set up to sweep! It relies a lot on prediction, and perfect scenarios, but even if I can just pass the acid armour without the sub, Mega Gardevoir is much more tanky and can survive pretty huge physical blows! The times it's worked (which is more than you'd think because people will try and set up against my Vaporeon, giving me an extra turn to baton pass), I can sweep up to three or four of their pokemon at the beginning of the round! I even OHKO'd a doubly sword-danced Lucario (not mega) in this way, who got me down to ~30% with bullet punch. The opponent then sent out a Noivern who used shadow ball, and I survived with ~5% and OHKO'd him too! The setup also works out well if I start with Vaporeon knowing that they're starting with a hazard setter, because it gives me more turns to set up. I use Vaporeon to baton pass to other pokemon as well, like Trevenant or Breloom as well, depending on the situation, so it's not really a waste of a team member either!
I know it looks like a grandiose and idealistic sort of setup, but tell me what you guys think of it! :D
(p.s. It's my first time posting here, so be gentle) :P
Edit: It's funny that EternalDream posted that bit about passing defense boosts, because while I was writing this he posted his comment.