User blog comment:SPARTAN 119/Chise (Saikano) vs Kongou (Arpeggio of Blue Steel)/@comment-1683193-20140101194539/@comment-379205-20140102000747

It is not made clear in the anime, however, the projectiles seem to directly impact her, and there is no sign of any sort of shield, so I would guess some sort of "unobtainium".