This achievement requires you to upgrade a weapon to +9. Upgrading weapons requires Steel. Each Level up requires 3x steel in order to level up so for example: to upgrade a weapon to +1, you need 3 Rank +1 Steel.
Steel can be found as loot, get dropped from enemies and is also given as a mission reward. From what i've noticed this basically aligns with the story mode missions (Mission 1 has Rank 1 Steel scattered around, Mission 2 has Rank 2, etc etc.) so assuming you don't switch weapons too often: you should unlock this before you finish your first run of the game. All upgrades can only be done by the Blacksmith who can be found at Mt. Tianzhushan.
I got this done after completing the first mission of Part 6, which i believe may be the first time you get access to +9 Steel. It seems like this achievement is buggy for some so I would recommend hard resetting your game amd xbox before doing the final upgrade since the game is Quick Resume eligible but that feature is still known to break achievements despite being a feature for like 3 years now