Yes, I agree it's slightly too powerful, but consider other combinations available to players already:
Plague/Retrovirus: perhaps not instant death, but for 2
it will clear all creatures that don't have either growth or immortality. Some creatures may of course live a while.
Thorn Carapace: see Plague, though small chance to miss.
Fire Shield: Ibid. Rain of Fire also available even in mono and will speed things up a lot.
Admittedly the above are slower. This new card holds advantages over 1) the above field-wide control, which take longer, and 2) Freeze-Shockwave, which is single-target.
So I'm trying to find a nerf that won't make it needlessly complex or fit a different element better. One possibility is to slay any creature missing at least half its health, this makes dragons more resilient and all but kills the Thunderstorm combo. It's just a bit too... weird.