Focus on Gardevoir, Shiftry, and Blaziken (I would train them til they are around 48-50 or whenever you feel like spongebob and scream I'M READY)
Gardevoir will be your MVP in the elite four. The more you train her, the better (if its a male...I apologize)
For Gardevoir, make sure she knows: Calm MInd and Thunderbolt
Also, buy some X-Speeds before the elite 4, they are handy.
Okay here is the strategy to win:
Note: Whenever you are in a situation where you are being outsped, use x speeds.
Sydney - Use Blaziken to kill his team, and use Shiftry/Gardevoir against his Crawdaunt.
Phoebe - If you still have the move shadow ball, use it on shiftry, his base 100 atk will help you out, a good strategy to sweep Phoebe is to use a 2-3 x-attacks on shiftry to then sweep her devilish ice beamin earthquakin confuse rayin dusclops and co.
Glacia - Use Gardevoir against jawalruses and Blaziken against her snowballs (x speed may come in handy, but idk its your team)
Drake - Now, this one is interesting since you dont have an ice beamer (not a fan of whale fortress) But here is the beauty of gardevoir, since drake starts out with a shelgon, you can easily set up a +6 calm mind since dragon moves are all special in gen 3) oh and use x speeds to outspeed his salamence, etc.
Wallace - I would say start out with setting up calm minds but his jamailor has double edge, which is no bueno against gardevoir, however you can easily thunderbolt and kill wailmer)...More than likely you will see either the dancing sombrero pineapple or milotic. Either way fouc on your calm minds, and heal when necessary. Now the beauty of this fights comes against milotic, if your gardevoir's ability is synchronize, you are set because milotic will poison with toxic since mirror coat will prove to be ineffective as long as you calm mind AND use x speed, it will be needed later. Once the pineapple, whale watcher, and milotic are gone, if you x peed and calm minded, you can easily sweep the rest of your team, especially with thunderbolt.