Why does Volcarona get increased damage when an opponent uses stealth rock?

Normally a move like stealth rock depends on the defenders type. To Pokemon who are neutral to rock type moves who work like stealth rock take 12.5% damage from the Pokemon just sent out. Which is about 1/8 of their HP(hit points). If stealth rock were to to have just one type advantage over the defending Pokemon, let's say an electric type like Zebstrika the damage will double. In other words Zebstika will lose 25% or 1/4 of it's HP. But Volcarona is a bit different. It's bug and fire type. You see, rock is super effective over bug and fire whick means it will do four times the damage! In other words Volcarona or Larvesta would lose 50% or 1/2 or half of it's HP. On the contrary, if a groud/water type like Seismetoad gets hit. Rock has a disadvantage over ground types so seismetoad will only lose 6.25% of it's HP or 1/32 of it. Avoid sedding out Volcarona when stealth rock is in play. Use moves like Rapid Spin to knock the move out of it's effect. P.S Use Stealth Rock to your advandage, don't get defeated by the move you have. Be prepared to defend and counter with a move like rapid spin.

Stealth Rock is a Rock-Type Move. How much damage that is done depends on a Pokemon's Type. Volcarona's biggest weakness is Rock-Type Moves, so a lot of damage will come from Stealth Rock. The more a Pokemon is weak to Rock-Type Moves, the more damage is done by Stealth Rock. The less a Pokemon is weak to Rock-Type Moves, the less damage is done by Stealth Rock.

Hope this helps!

Volcarona is a Bug/Fire type, which gives it a double weakness to Rock moves. This double weakness causes Volcarona to lose 50% of it's HP when it switches into them. Any Pokemon with at least one weakness to Rock will lose 25% of it's HP, while those that resist Rock will take significantly less damage. If you want to keep that Volcarona alive, you're going to need some Rapid Spin support.

Stealth Rock deals damage based on typing, like regular attacks. If a Pokemon takes neutral damage from rock type attacks, it will lose 1/8 of it's HP when it switches into Stealth Rock. Volcarona has a double weakness to rock type attacks, so it takes four times as much damage, for a total of 1/2 of it's max HP.