Bro, I suggested you watch my replays and test this team out first. It's not easy to use unless you're me.
Analysis:
Ok, I understand why you risked and switched in Altaria-Mega the first turn. You want to absorb the Trick in case it's not a Mega Gardevoir. That game changing risk was simply and absolutely unnecessary. The best and safest play here would to switch in Doublade. So what if Gardevoir is Scarf or Specs... Doublade's main purpose here is to tank hits from Mega Gardevoir and Mega Medicham... so even if it's Tricked an item, it wouldn't hurt the team one bit. Regular Gardevoir cannot and will not break Chansey.
I would Toxic here on the obvious Stealth Rock instead of using Scald. If the opponent switches in Skarmory, then Scald. Five of his 'mons except Skarmory are weak to Toxic and would be on a timer when inflicted. A burnt would be good too regardless.
Why Toxic Skarmory again? I'd Scald or switch in Mandibuzz here. Scald would be the better move in case the opponent Whirlwind the Mandibuzz switch in. Either move is 10x better than Toxic though.
Why switch in Mega Alteria to eat Spikes and Stealth Rock and a WoW this turn??? Just click SCALD.
This was an excellent move staying in to Defog against Raikou on the Gardevoir switch. A comeback could happen if played properly from here, the game could still be won here. It was not over yet.
Turn 30 makes my heart bleed, and it's still bleeding. Why switch out Chansey? YOU NEED CHANSEY HEALTHY HERE; SHE IS IN KILLING RANGE FOR MEGA GARDEVOIR. The game is pretty much lost. The correct move was to Soft-Boiled and then Stealth Rock. That's clearly a TankChomp and its Earthquake does 30% to Chansey max.