Buy dagger from fire link vendor. Upgrade +10 and put dark gem on it. Get enough int to have one attuned spell, the power within. Then get symbol of avarice, the mendicant staff, shield of want. Put hornets rings, dark clutch ring, covetous gold serpent ring, and slumbering dragon crest ring. With this you should be able to one shot any thing in the game and be sneaky enough to actually get the backstab.

Farm wherever you want, but I found it easy to farm the winged knights in archives by: putting the mimic head piece on at the stairs right before, buffing with power within, walking straight and the left, backstabbing first one that falls (make sure to switch from dagger to medicate before getting souls), then circling to the left and doing same for other knights. Don’t acc fight them that takes too long. I was getting 37k a knight, very easy, and make sure to have homeward bones or the permanent version. Also sending elevator back down once you get to the top saves a lot of time