Out of all the games i found gen 3's elite four to be the toughest. I would stock up on the hyper potions and revives.
I would teach Castform a electric type move to get past Glacia's Sealeo and Walrein. Groudon shold know fire blast should defeat her glalie's.
For Drake i would use castforms hail and id replace powder snow with either ice beam or blizzard if you have them. If you had taught it an electric type move that would work too on Altaria and Salamence.
For Steven the champion start out with Castform again and use either ice or electric type moves for Skarmory. Swampert's water moves should take out his Aggron and Claydol. Groudon should be able to deal with Cradily as it is part grass type. Groudon and Swampert would take Armaldo and Groudon for his Metagross.
Plenty of Hyper potions and Revives will be needed and ethers would be helpful if you have them.
"over the laws of light, over the moon by midnight"