Sp.atk and Spd EV's
Heat Wave - STAB, powerful and a chance to burn.
Hurricane - Very powerful and a chance to confuse opponent.
Bug Buzz - STAB, powerful and a chance to lower Sp.def.
Giga Drain - Good power, coverage and health restore.
Okay so this set for Volcarona is pretty good, especially due to its amazing Sp.atk stat, and volcarona also has impressive speed and Sp.def stats, truly a powerhouse, giga drain covers two of volcaronas weaknesses especially rock, plus most fighting Pokemon can use rock attacks, so hurricane is an extra precaution, especially since volcaronas defense stat is average, so this set covers all weaknesses except flying, which is okay because you could probably knock it out in a shot, or at least severely damage it with heat wave, rock attacks are the main thing to watch out for, especially since most of them deal physical damage. Swarm helps boost bug buzz, making this Pokemon a sweeper of note, especially with its back against a wall, a very good addition to your party, and an unstoppable one if EV trained correctly, please note that it is female to counter act Pokemon who use attract, as most female Pokemon users equip them with attract, also wide lens covers the accuracy gap in heat wave and improve the accuracy gap in hurricane (please know that hurricane receives full accuracy in rain, 50% accuracy in harsh sunlight and is capable of hitting Pokemon using fly or bonce in its invulnerable stages).
P.S if you are worried about the flying weakness, you can replace hurricane with hidden power (ice, rock, or electricity) or alternatively, replace giga drain or hurricane with quiver dance, it raises speed sp.atk and sp.def all in one go, volcaronas best three stats.