Why cost so much for only one turn of blockage? I feel the cost could stand to lower a couple of quanta.
4|3 might be better, but I'll explain what I was thinking.
Compare it to Dusk Mantel | Improved Dusk, which is 6 | 4.
For the first turn of damage, this shield blocks 100% of the damage, so it is twice as effective as Dusk Mantel. On the second turn, it is equal. On the third turn it is 33% vs. 50%, which is still not too bad. So the gamble for this card vs. Dusk Mantel is that you will be able to counter any creatures 1 or 2 turns after they are played. In a short game (7-8) turns, it is likely that the game will be over 1 or 2 turns after some creatures are played.
This shield is designed to be optimal against low HP expensive creatures that can be taken out with bolts. It is a soft counter to the squishy dragons, with general utility in any rush deck.