Just discovered this in an attempt to replace a cart battery for a kid in the neighborhood...
Circlip pliers, commonly used for installing/uninstalling snap rings on engines. Example below:
Grab a hold of the Game Boy cart screw with the pliers and rotate the cart, not the pliers. Spins `em right off without stripping the sides.
Circlips are probably available at Home Depot or Lowe's. You can get them with straight ends but mine are angled like in the pic. I bought my pair at a motorcycle shop for $12. That's still cheaper than getting the specialized bit from Lik Sang.