I have a 12x20, similar to yours my ground was mostly level. I dug out and flattened an area 1 foot bigger than the shed. I removed all the top soil down to the clay and compacted dirt and rocks, luckily this wasn't terribly deep for me, I think 8 inches or so. I roughly leveled it and rented a compactor to pack down what I moved. I used landscaping timber for the area that needed raised up, the areas under grade were left alone. Nefore i hadad my stone delivered I soaked the area it was going to soften the ground and used the compactor on it. It has been 2 years and it hasn't really sunk. I had some mild settling after the first winter, just had to jack one end up and move some stone around with a rake and I was good to go.