Well, I really don't think it is OP. I suggested the HP rule purely for thematic reasons.
EDIT: I hadn't checked the cost. It might be slightly OP.
EDIT 2:
However, dragons are airborne, so sky blitz could double them. That could do multiple in one turn.
EDIT 3:
Now that I think about it, something should make sure this doesn't stack with Sky Blitz.