I have struggled with this for the 22 years I have had my runway. If winter freezing is an aspect, there are added complications.
The best solution, which I have done on nearly half of my runway, is to plough it up (farm field style), and grade it back smooth and level, with a crown, and ditches both sides. This will produce the best result. If that is not possible (rock close underneath), bringing in loads of soil will be necessary to fill the low spots, then grade that in. To be effective, the machine used to grade the soil must have the blade midway between the front and back wheels. Though it is possible to grade with a front or rear mounted blade, it is very much harder to get the ground level. The friendly farmer will tell you he can do it with his blade on the back, or back dragging the front bucket, but you won't be happy with the result. You'll know you've got it right, when you very happily drive an expensive low slung car back and forth the length at 100 KMH.
Different aircraft types will handle rough ground differently, with high wing singles generally being a bit more forgiving, and some more than others. Over the long term though, even though the plane might handle the bumps, it is still hard on the airframe. Better to fix the ground first, that the airplane later!
Pilot DAR