Ok a lot of people have been saying 2-1 to farm for SG 12 but i will give another suggestion run 1-3.

Now there is obviusly the question about "why this crappy stage?" Well... that pretty simple because even if you are more efficient with 5-4 or 2-1 1-3 just gives you a little efficiency in gold exp and whatnot, while 5-4 uses 2 much stamina to actualy farm a lot of manyus which you need and the 2-1 is a little more efficient while consuming 3 more stamina each run.

Well everything is up to you if you want to have more efficiency i suggest running 5-4 but if your only interested in manyus farm 1-3 or 2-1. In every case you will need double the exp you get from the maps if you want to level after 40 so before that you can just farm leisurely everyone of those maps.