Sounds about right... though maybe just slightly on the UP side.
A typical game only lasts about 8 - 10 turns. And, at least in non-upgraded battles, most creatures won't show up till at least the second or third turn. That means creatures will only be in play an average of about 5 to 8 turns...
So this card will only ever compete with firestorm at the very last turn or two of an average game.
So with respect to firestorm this seems about right... on the other hand, compared to thunderstorm and dessication, this is very expensive.
I think a slight cost reduction may be feasible. Something somewhere between firestorm and thunderstorm is probably the right place for it... ( 4 to 6 unupgraded and 3 to 4 upgraded).
The scaling seems pretty well balanced though.
On a different note, time is an unusual element for direct damage spells.
Typically the time element is more about delay and card manipulation and tends to rely on non-damaging types of CC. This seems to be one of the defining aspects of the element.
This card does introduce direct damage in a thematically interesting and relevant manner though, so that should count in favor of an exception.
