"Fire at her on my count!" shouted the leading voice. "One, two, three! " and maybe-forty voices roared spells, creating a huge concentric array of fiery bolts that lit the wide corridor brighter than the Sun -

for the short moment before the bolts struck and vanished upon a dark red octagon that appeared in the air around the girls, and then disappeared a moment later.

Hermione saw it, she saw it but she still couldn't imagine it; she couldn't imagine a Shielding Charm that powerful, a spell that would withstand an army.

From Ch. 74. It's not clear to me whether the shield was invisible until struck or if he put it up very quickly and silently.

We've also seen him just flatly stop curse bolts in midair and then flick them away, without apparent shielding or obvious effort. He's got defense options like Smaug has gold coins.

If killing him was easy, someone would have done it before. Even though he had horcruxes, it's telling that he never actually had to respawn from one until he tried juggling dynamite and blew his own self up.

This.