This trophy is unlocked the first time you upgrade any weapon to level 4. Only one weapon shop is able to upgrade to level 4--the others only upgrade to level 3. So, there are two conditions that must be met to unlock the achievement. First, you must be able to pay the upgrade price in gold and materials. Second, you must have access to Master Swordsmith Masamune's forge.
A logical weapon to upgrade is Virtuous Contract, 2B's default weapon. Assuming that you have already upgraded it to level 3 at the Resistance Camp Weapons Shop or the Machine Village Weapons Trader, you will need the following to upgrade it to level 4: 2 Memory Alloys, 5 Severed Cables, 3 Pristine Cables, 2 Machine Arms, and one Moldavite.
All the materials except for the Moldavite can be farmed in the pit where you encountered Adam & Eve the first time as well as at other locations. The Moldavite can be found fairly commonly at harvest points near the Flooded City save point and less commonly at the Forest zone save point and other locations in the game. If the harvest points do not, at first, yield the Moldavite; make a quick save and then load it. The harvest points will respawn. Continue harvesting in this manner until you get it. Moldavite is a reward for several quests including Wandering Couple that can easily be completed before finding Masamune's shop.
You can find Masamune's forge while undertaking the Forest Kingdom Main Story Quest. It is close to the end. When you interact with him, you will have the option of upgrading weapons. Upgrade any weapon to level 4 to unlock the achievement.Caution and recommendation
: This achievement glitched the first time I attempted it. There is a save point on the main route just past Masamune's forge. I strongly recommend making a save here before you attempt unlocking the achievement. Defeat the enemies close to the access point to make it available. Then, make a save at this point. This save point is the Forest Castle: Royal Chamber
access point. Once you use the save point, map information will appear including the location of the nearby forge if you have not already discovered it. Go back a very short distance and drop down one level as soon as you get to the edge. Interact with the block and pull it back to gain access to the forge. Upgrade a weapon to level 4 to unlock the achievement. If the achievement does not unlock
: Press the
on the console for more than ten seconds to shut down the XBox. Unplug the power for about 20 seconds. Plug in the power and start up the XBox. Load the save you made at the Forest Castle: Royal Chamber access point. Go to Masamune's forge and upgrade a weapon to level 4. The power cycle you did on your XBox will have cleared the cache and the achievement should unlock.