Over the years I have developed the strong view that a lot of things that can be fiddly or otherwise annoying to do standing up are much easier if you sit or kneel. E.g. unloading a tube magazine on a pump shotgun, or cocking powerful springers.
My technique for pumpers is to sit or take a knee in conventional fashion (left knee up, right down, for a right hander).
Hold the gun in front of you with the right hand side facing you.
Place the heel (top) of the butt in your left instep.
Place the palm of your left hand on the top of the barrel.
Use your right hand to operate the pump handle.
For the higher pumps, you can use pressure from both arms.
It works for me. Far better than trying standing up, either holding the scope (bad idea) or the pistol grip (poor leverage).