Since your asking for Manyu's 2-1 is best place
Since you only at stage 9, you will get alot of lvl 5 and lvl 20 Manyu's, though you get alot for the stam cost, this will cost you alot of gold in the long run to use the Manyu's.
so try to push to stage 12 if you can to farm 12-3 or 12-5, admittely most people farm this stage for the acceptable drop in Ampies and not for Manyus.

The above is really for spawning the secret garden's. Can not remember what stage the 80 stam secret start spawning, that is considered better as it's the only stage that drop LVL 100 Manyu's, but at 80 Stam each, you might find it expensive stam wise, but i guess that the trade off.

If you find it's way too hard to farm 12 then not much you can do to improve your situtation.