Forget Hidden Power Electric. Unless you are running Choice Specs or you are really aiming to take down Gyarados, Hidden Power Electric is simply way too weak to deal meaningful damage. Same goes for Hidden Power Grass vs Gastrodon. If you really want to take down bulky waters and hurt certain Dragons, Toxic is definitely the way to go. You have to remember that most bulky Water-types can't do shit back to you because of Water-Absorb. I remember back in BW some Keldeo ran Toxic on Choice Specs sets in their free fourth moveslot to screw over Jellicent and the like. Anyway here is what I am seeing:
Volcanion (M) @ Leftovers
Trait: Water Absorb
EVs: 160 HP / 252 SAtk / 96 Spd
Modest Nature (+SAtk, -Atk)
- Substitute
- Steam Eruption
- Fire Blast / Flamethrower
- Toxic
With its great set of resistances and solid bulk, Volcanion is surely going to have an opportunity to get up a Substitute. Both Steam Eruption and Fire Blast can deal out major damage backed by base 130 attack and STAB, although you can use Flamethrower for more PP and consistency. Toxic screws overs any bulky Water thinking they can outlast you. Behind a Substitute and Water Absorb, you will be sitting pretty. Dual status with Steam Eruption and Toxic will surely put a lot of pressure on your opponent.
Overall I think Volcanion is in a Kyurem-B like situation. At first its movepool seems really shallow, but if you examine it more closer you will see it is a much bigger threat than expected and has more to offer than its movepool appears to be. Choice Specs is looking really good (esp. with Overheat), and defensive sets can easily be pulled off with Roar, Toxic, Wil-o-Wisp, Haze, and Steam Eruption at its disposal.