This achievement requires you to upgrade a piece of armor to +9. Upgrading weapons requires Leather. Each Level up requires 3x Leather in order to level up so for example: to upgrade a piece of armor to +1, you need 3 Rank +1 Leather.
Leather can be found as loot, get dropped from enemies and is also given as a mission reward. From what i've noticed this mostly aligns with the story mode missions (Mission 1 has Rank 1 Leather scattered around, Mission 2 has Rank 2). This does taper off near the middle of the game but you should be able to find +9 leather by the time you reach Part 6 so assuming you keep 1 piece of armor with you to the end, or maybe just horde upgrade material until you get close to finishing the game: 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.
It seems like this achievement is buggy for some so I would recommend hard resetting your game and xbox before doing the final upgrade since the game is Quick Resume eligible and that feature is still known to break achievements despite being a feature for like 3 years now