That energy field would make a good shield, as well as a good attack, and Sin uses it for both. Sin first uses it while not being attacked a moment before the Al Bhed fired upon it, and used it as a weapon when it did so. When the Crusaders and Chocobo Knights are battling Sinspawn on the beach, Sin formed the energy field around itself and then fired it toward the beach, vaporizing its Sinspawn along with those battling them.

Then, the Al Bhed power up their large weapon and fire it at Sin just as it had formed another of those energy fields. Like with the first, Sin pushes it forward and dismantles the Al Bhed weapon.


It's not that it was a shield or that it was just an attack. It was just energy that Sin could harness as either.