Purchase a house with grass in it. Then, go to the Belle Isle Garden Shop and purchase some fences that will cover the front part of the farm. Align the fences. Purchase bombs, then throw them on the grass. Do not collect the Gralats when the appear. Instead, let them stay there and keep throwing bombs at them. They will begin to stack in value and turn into other colors. Their values are as follows:
Green: 1 to 4
Blue: 5 to 29
Red: 30 to 99
Gold: 100 to 999
Purple: 1000 to 9998
Black: 9999 to Infinite