You don't need a "case cracker" or anything like it. I have taken apart countless Compact Macs with this very simple technique:

Lay it face down on a towel, remove all four screws. Then give the sides of the case an "upward slap" (for lack of better words) with both hands at the same time. This will free the rear bucket from the front and allow you to simply lift it off.

60% of the time it works, every time. No cracker required, and no risk of damaging the case. The "slap" is also not violet enough to damage or dislodge anything inside the computer either.