Have you ever tried to use something besides just big rocks for your staff sling?                                                          
     One of the staff slings biggest problems is that it is harder to aim than slings or slingshots or bows. My solution to this was to get a big pouch for the sling and load a handfull of pebbles or chips in. When you shoot it, it is like a shotgun and a hailstorm of rocks flies down in the direction you shot at! Now it's easier to aim.
    You might want to try shooting ice chunks instead of rocks because they can be molded and are more expendable.

Or when its freezing (literally) outside take a rock and stick it into the water. It gets a layer of ice around it. Keeps puting it in until its covered then shoot it and when it hits something the ice around it blows up.