[MikeSid]

An electric jackhammer will make pretty quick work of that.
If it is just dirt formed, there is likely no rebar and they didn't vibrate it. So it should break apart into large chunks pretty quickly.

Since you are renting, pick one sharp pointed tip to use. You may be charged for sharpening each tip you use, so pick the pointed one and stick with it. Use the weight of the hammer, eye protection and steel-toed boots.

Given the pic you posted and the way the ground lays, be sure to stand above the tip of the hammer. It may sound obvious, but don't have your feet below the level of the tip of the hammer at any time. Looking at the pic it would seem tempting to stand with a foot in the hole. For reasons which should be obvious, that's not a smart move.