(You should still have a spoiler. He may not be the only one reading the thread, genius)

(SPOILER)Actually, I didn't think it was a shield. I thought it was Sin's attack. Sin attacked at the same time as the machina did. The two attacks basically fought against each other (imagine two guys with swords attacking each other. They can each hold the other's weapon at bay until they run out of strength. Sin and the machina just used energy weapons, but the same principle applies), until the machina broke, and its attack collapsed. Sin's attack then burst out unhindered and destroys the machina and obliterates the Crusaders and Chocobo Knights. That's why the attack hit at the same instant the machina could no longer keep up the pressure. The shield was Sin's attack. Just my opinion on the subject, mind you.