Under the sun, pair Volcarona with a powerful physical Attacker such as Darmanitan, as wel as a Chlorophyll sweeper such as Venusaur, and there really isn't anything that can stop your onslaught. Sun teams play similarly to heavy offense, except there's Ninetales instead of a Dual Screener (although many Sun teams carry Espeon). Often times, Volcarona doesn't even need to pull off a sweep, it just needs to weaken the opposing team enough so that someone else can break through.
Oh, and just for lols, a resisted Fire Blast in the Sun has essentially 135 base power, approximately the same as a super effective Hidden Power. That is why Hidden Power Rock honestly isn't useful at all, the imperfect Speed IVs just adds another negative to the pile. Hidden Power Water is potentially useful in the Rain, as it'll have more power than Fire Blast. However, Rain abuser Volcarona's best trait is perhaps its ability to take out Sun teams, so Fire Blast is usually a better option.