I got this achievement for Level 6 Forging (and its Level 4 Forging predecessor) as early as Burgusstadt using Edward.
In Luce Plains, farm the enemies as much as possible (repeatedly killing the Box Mimic beyond the cracked rock wall helps this to go faster ... after you get Balbagan in your party) and save up quite a bit of Fol.
Head to Burgusstadt (after completing Castle Prevant) and go to the General Store, buying as much Bronze Metal as you can carry. Go to the castle. If you have any Metal Fragments, visit Sigmund in his room, connect with him, and turn all of the Metal Fragments into Bronze Metal.
Now go to Edward's room across the hall and connect with him ... but instead of sitting down and churning out weapons and armor right away, have Edward follow you back to the General Store (this saves a bit of time from running back and forth ... waiting for the slow ass double doors to the castle to open each trip).
Inside the general store, start churning out Bronze Scale Greaves until you reach Level 4 Forging. Once you have Level 4, start turning those Bronze Scale Greaves into Bronze Plate Greaves.
Rinse and repeat ... selling the Bronze Plate Greaves and buying more Bronze Metal when necessary ... until you get Level 6 Forging.
This nets you quite a bit of change. After I attained Level 6 Forging, I continued mass-manufacturing the Bronze Plate Greaves, saving up a lot of Fol, then purchasing more advanced materials (you may or may not need the Vouchers DLC ... not entirely sure) and making some of the more advanced gear for people in my party.