I'm not saying it's tough.
I'm just stating how I hit half volleys, at a high 4.0 doubles level.
As a very skinny and weak person, I never tried to hit topspin on my half volleys. No need to. Good placement with some side/slice makes a good enough shot to force the baseliner to run full speed to get to the ball. If he crushes a clean winner from there, he can hit the same winner if I added a little topspin to my shot.
Don't think I'll ever need shots to face Sampras or Federer.