I don't know why everyone is hung up on the "it should be invented by now" and are missing how just like anything with the show lately it's just conveniently discovered just as needed.
It's even the same plot device they used last season. "Oh Cersei you are in danger? Here's a solution I just happen to come by". And this time we even got a scene with them targeting the skulls of a dead dragon, to make the dumb and obvious writing even more obvious.
For a show that cared so much about details and actually developing stuff for more than just the next few episodes this is very disappointing. Had they about two seasons ago had a scene with the science guy asking permission to develop new weapons, and then last season had a scene with him showing some of them (without the dumbing let's shoot the dragon in the head) the payoff for them actually using the weapon now would be so much better.
Edit :
Jesus, next season will be even more rushed then?