I like the idea, but I feel like it's a bit too efficient stat: cost wise. You can't really drop the defense without defeating the purpose, but maybe raising the cost by 1?
For 4 quanta (2 supernovas, or pends or whatever.) 16 HP shield that also damages opponent.
For 6 quanta, 25 hp shield.
I think, for half the defense of 4 quanta, and a different effect (targeting instead of creature attacks), the cost is fine for the defense.
Additionally, it's a weaker form of Reflective Shield, which also costs 2 quanta and protects the owner from a whole bunch of spells. This one protects some creatures for only 8 hp. 2 lightnings can take it out.