It is a maze just like the last answer said. You must get the National Dex first, though.
Volcarona is inside Relic Castle.
In relic castle
You can get Volcarona by catching it at Relic Castle or making your Larvesta (if you have one) evolve into Volcarona starting from level 59.
you can find volcarona in both games in relic castle after beating the elite four and this Pokemon can be boy or girl also its first form is found in a form of a egg on route 14
Breed the volcarona that you find in relic castle (with a ditto if needed)
In Pokemon B/W you need to go to relic castle or get larvesta on route 18 and get it to level 59
in terms of levels, the strongest pokemon in the wild is a lvl 70 volcarona in one of the bottom rooms in relic castle after u have beaten the main story once
yes when you go find one of the sages at relic castle when you are almost where ghetsis is you can battle onix and sandslash. after you find ghetsis you can also fight volcarona
relic castle
you can get the egg near nuvema town and you can catch it's evolution volcarona on relic castle the deepest part
Relic Castle in the desert
you have to get to the bottom