I usually hold the punch in my left hand, with my fingers going through among the spokes, so my left hand is both holding the punch and stabilizing the wheel. The wheel would be resting on my workbench, and perhaps also leaning against my chest. I've never needed an assistant for this.

The O.P. is going at this in the wrong seqence, however. All the stuff from the _left_ end of the axle should be unscrewed, but then you use the old hammer and punch on the right-hand ball ring to remove the entire "innards" as a unit.

I have modified factory instructions at: http://sheldonbrown.com/sturmey-archer/service

Sheldon "Epicyclic" Brown