Are you able to get a cement truck back there, or are you mixing by hand?
There are a few options I see. Level the fence line with a shovel about 10" wide, set post around 30-36" deep and place railroad ties between them. Drill the railroad ties and pound in rebar 24-36" deep every few feet.
Level fence line, use dry stack interlocking retaining wall block and set the fence back 8" onto your property.
Level fence line, dig your post holes, then set concrete forms. Use maybe a 2x10 on the down hill side and a 2x4 on the uphill side. With the posts 30-36" deep every 6-8', it should not move.
I have been using 2 7/8 drill stem for my fences using the field fencing. So far, 3 pigs have not been able to get through it.