There's a few ways to work this out. But do you really want to know impact energy, force or stress? Why?
You'll need to know the contact area to calculate the stress, the target stiffness or deflection to calculate the force, and you'll need to know the piston travel in all cases to know the energy. That's all assuming your robot chassis doesn't take off/move during the swing, as that can absorb a significant amount of the swing energy. For a low energy hammer that's a fair assumption. for anything competitive, it significant and only a complex model will tell you what the result will be.
How much energy you've got to play with is fairly easy to calculate: piston force x travel = Energy. (force in Newtons, travel in metres)
Hammer head weight can come into it a little too. If its a short, light hammer, then the piston will be running at maximum speed, limited by the speed the gas can get into it, so the full energy won't get transferred