There is very little RNG here, even though it's got the word "random" on it.
In the vast majority of cases, it's going to clear the board, period. If the Shaman has minions, he's going to trade them before playing Volcano. And since it can't hit face, there's nowhere for excess damage to go.
Provided your board is empty, this card is considerably stronger than flamestrike. 15 damage will usually clear pretty much any board the apponent has. Plus this card works as a secondary hard removal for big minions.
it removes 15 health from the board guaranteed. That seems pretty reasonable to me.
I mean, it's basically a slightly different spin on Elemental Destruction. The decks that would use Elemental Destruction would use this. But since Control Shaman isn't a thing we don't think about it.
If control shaman becomes a thing, you can bet this will probably make the cut.