Details of the making of this mod.
To get started, I wanted a 100% drop rate mod for Battle-Tested, but there were none. So I decided to make it myself.

The first resource I found was: https://www.youtube.com/watch?v=_EhwNMKmqHA
While it isn't for the second game, the process is quite similar, and there's a link to the modding discord in the description of the video.
Trust me, you'll need it.
Okay, so, the entire basis of this mod is the need to change the Octopath_Traveler2\Content\Paks\Octopath_Traveler2-WindowsNoEditor.pak file.
Copy and paste this file somewhere, and use an application to unpack it. There are many different ones you can use, and many are available in the modding discord's #resources channel. The unpacking process takes a while, depending on your PC, so if you want to, say, change only the enemies and don't need to unpack the entire thing, check the modding discord's #file-dump channel to see if they have what you need. In my case, there was an Enemy.zip file with the needed EnemyDB files, which hold the droprate information for enemies.
Using https://github.com/atenfyr/UAssetGUI and one of the OT2 spreadsheets of Townspeople information, I was able to locate the Townspeople that had Battle-Tested weapons in their drop repertoire. Scrolling through UAssetGUI, I moved through the various packets of information until I found the correct ID, at which point I would scroll down until I saw 'Droprate', and after confirming that it was indeed 2 (the default rate), I changed it to 100. After saving the file, I ran into what would be the most annoying problem: changing the EnemyDB flies into a .pak file.
After trying various applications, I settled on https://www.fluffyquack.com/tools/unrealpak.rar
But it's not over, no no no. If you want your mod to work correctly, you'll need the proper file structure.
The file structure has to be the same structure as the actual Octopath_Traveler2-WindowsNoEditor.pak file.

For example, here was my file structure:
GuaranteedBattleTested_p\Octopath_Traveler2\Content\Character\Database
Since the EnemyDB files reside in Content\Character\Database, I matched them in my file structure, and put them in the Database folder.
After dragging GuaranteedBattleTested_p to the Unrealpak .bat, it was turned into a .pak, which would be the mod itself.
(Adding a _p to the end signifies to Unreal Engine that the .pak file is a patch, or something to that effect. Google for more information, I guess.)

Unfortunately, because the .pak holds literally every enemy, it is incompatible with other mods modifying the EnemyDB file, as it will directly override other .pak's that might change a certain enemy with the default information. But what can you do about it, eh?