Go to Relic Castle. If you haven't been back here since beating the game, Professor Juniper will appear and give you a RageCandyBar. If you give this item to one of the statues around the entrance to Relic Castle, it will attack you in battle. It's a Darmanitan that has the Zen Mode ability, which transforms it into a statue when its HP gets low. Catching one of these is the only way to get a Darmanitan with Zen Mode, so be sure to catch one.

Relic Castle Revisited

Go all the way to the bottom floor. There used to be no way to go farther, but now the sand will let you reach a door that you couldn't get to before. You will soon run into a Plasma Grunt. Follow him. He will fight you with an L64 Krookodile.

Go south from the Plasma Grunt and keep going southwest until you find Ryoku, one of the six sages. Talk to him to get TM04 Calm Mind.

Go north from there to find Volcarona's chamber. You will encounter Claydols here.

Catching Volcarona

Volcarona is at level 70. Its catch rate is lower than Zekrom/Reshiram's, but higher than that of a legendary pokémon. The best pokéball to use is the Timer Ball if 30 rounds have passed, or the Dusk Ball otherwise. The next best is Net Ball.

When you catch Volcarona, you can exit its chamber and head northwest to eventually reach the western tower of Relic Castle. Your Dowsing Machine will find a PP Max along the way. Once you get to the tower you will find TM26 (Earthquake).