This actually seems a very good card idea to me. 5f seems about right, or maybe slightly high, not sure. Is it 2 mountains?
You might want to add “to enemy creatures in combat” so that damage from Crackthorn, Flamespitter, Groundshaker, Plague Bearer, etc isn’t doubled or ambiguous.
I can’t think of any OP synergies. Deepwood stalkers still count as combat, but that seems ok. Buffs get doubled, but that seems fine to me. Haste creatures would go nicely with this. Most of its synergies are off-color, not sure if that’s a good or bad thing.
It’s not a total giant annihilator, but it seems that’d be its best use.
Another question - how does it stack if you cast it twice or more? 4x/8x? Still seems fine to me.