Phasing.
It is implied that Saitama is indestructible; however even if he is, that is irrelevant to Shadowcat (from X-Men). She can phase through the skull of her opponent, including indestructible characters like Wolverine, and fuse/squish their brain with her hand to instakill. (What If: Wolverine - Enemy of the State #1)
By making herself intangible she can avoid any attacks; so long as she maintains her intangibility she can just wait until the right moment.
