Here I'll explain how to perform easy edits that can drastically alter how this soldier works.
Only bother if you:

A. Want to remove or edit permanent stat loss or gain.
B. You want the Ace to be more or less powerful.

And here's what you need to do:

Step 1: Go to...
C: or D:\SteamLibrary\steamapps\workshop\content\268500\1695897994\Config\XCFAce_LW2

Step 2: Open up...
XComGameData_SoldierSkills.ini using Notepad or something similar.

Step 3: Find what you want to edit and simply change the number. See example below. Don't forget to Save after you make changes.

Examples:

A. Temporary Aim loss only.
Change: TemporaryAimLossAtStart=-20
Result: Now all allied soldiers lose 20 aim at mission start for that mission.
Change: AimLossAtStart=0
Result: Now no aim is lost permanently.

B. More Difficult Solo Operative Missions
SoloOpRefundsPerTurn=2
Result: Solo Operative ability only grants 2 refunds each turn instead of 3.