NOTE : THIS IS NOT A COMPLETE GUIDE No one has created a complete guide for this Achievement yet! Only read on if you want a tip that helped me get this achievement that I am sharing here.
This solution is for 1 potentially easily missed Weapon Upgrade Kit ONLY
Pickup location is on Eva'a Hammer.
Note : As stated this is a guide/solution, for only for 1 possible easily missed location. If this did not help your individual situation please do not unfairly cast a negative vote without leaving a comment why.
You need a total of 21 Weapon Upgrade Kits to fully upgrade the 7 Weapons.
Most are easy to find through normal game play, and can also be tracked in the Districts on the War Table in the Helm. But if you were like me and finished the game with only 20/21, here is the location that got me the achievement.
The one I missed and finally found was located in Set Roth's Kill House, which unlocks not too far into the game. This is where you need to be quick and complete the Kill House in under 1.29 minutes for the First Loser Achievement.
So if you were like me and ran through this challenge with eyes on the targets only and didn't think to replay or go back and investigate the area, this is were you will find another Weapon Upgrade kit waiting for you. It is on the right hand side near some lockers in the very first section of the course after the first target.
Hope this helps someone.
Note : As stated this is a guide/solution only for 1 possible easily missed location. If this did not help your individual situation please do not unfairly cast a negative vote without leaving a comment why.