If Diana Fails to One-shot, is she guaranteed a death?

Diana late game can one-shot, but say she tried to oneshot a champion that weren't so squishy. Her E has a rather long cooldown and Her W will no longer be effective if they run away. She only has her Q and R. You could wait for your Q, then use your R, but they have lots of time to escape, or you can R straight away.

So, in this case, if you were playing diana and couldn't one-shot them, and they started attacking you (Let's imagine theyre a slightly tanky AD), Are you pretty much done for?