Azir's Ult
There's still so much buggy about this but one of the for-sure things I can say is Riven > Azir's Ult. I used it to her twice to peel her away from me and it just doesn't knock her back. I'm not sure if her skills are supposed to make her immune to that type of knockback, but it didn't even 'kind-of' knock her back (like when a dash and knockback collide and the champ sort of goes nowhere) she just walked straight through it.