They just refuse to understand.
Cap has no idea what these death lines and points of existence are and where they are, so he'll have no idea shiki is aiming at.So yeah he'll stand there like idiot not understanding what she's doing, because he won't realize the sheer godmod that is shiki
She can do a multitude of things
such as erase his friction and halt his movement as wolf has said
erase the existence of his thoughts and thus his battle knowledge or his current strategy, i'm quite sure he'd be disoriented by that suddenly happening, no matter if he's marvels greatest hero
if he ever throws his shield, then he loses it forever, because it will be destroyed. cap flanking isnt a problem when she can just suddenly warp behind him
this girl can warp around everywhere, and you question her speed?