What you need to do is shoot and use Snatch when you jump in the air to dodge the electricity.
You will be able to stay in the air, stun, even knock down the swords, or you will be able to bring a sword closer to you then if you Buster right away you will stay in the air as you pick up the Gladius and throw it at Agnus.
Also, stay in the middle, seeing as the electricity comes from the middle thing, and for some reason not out of the top of it.