How important is Void Staff in this situation?
I was a kassadin with

i caught soraka behind her team
she had

Yet i couldnt kill her.
This is what happened
Did 1 rift walk to get to her ( didnt land ) Landed Q + W no E and landed a 2nd stack riftwalk on her + auto.
Im not saying why didnt i kill her, im saying how did i only manage to do 30% of her health. She used locket shield but still she was by herself and i was like wtf i should do more damage than that.
Is it because no void staff against locket 60 mr + merc tread 25 mr = no damage? Is void staff really that big of a deal that early?