Q:

How to delete pixels without having to click multiple times? 

If I try to delete some pixels, then I select rubber tool and set hardness to 100%.  However, I still have to click about 4-5 times at a pixel so that it completly removes and turns to alpha. 
Why is this the case and how can I solve it? 
It does still not delete all inside my circle on one click , even if everything is on 100%

A:

It sounds like either the Flow or Opacity has been reduced.  Make sure both are at 100% and then it should work on a single click if you also have the hardness set to 100%. 
update:
Given the new comments and screenshot change your Eraser's mode from Brush to Pencil and then it will work how you want it to.