Hard. The final Riddler trophy. Steam save.
You just need to collect the final "Riddler" trophy and then go into battle.
This isn't "NG+."

Just in case, disable Steam cloud saving and "log out" Steam from the taskbar.

Extract the files to any available folder.

Copy all the save files (4) and paste them into these paths, replacing them. Delete any files that weren't replaced.

D:\SteamLibrary\steamapps\common\Batman Arkham Knight\BmGame\SaveData
C:\Program Files (x86)\Steam\userdata\353912931\208650\remote
C:\Program Files (x86)\Steam\userdata\1340374462\208650\remote

Delete backup saves from here just in case. Also, just in case, make a copy and move it to another shared folder.

C:\Users\YOUR PC NAME\Documents\WB Games\Batman Arkham Knight

Launch Steam, then the game itself. If my save file is installed, play for a minute, then exit the game and enable Steam cloud saving (if there is a file conflict between your PC and the cloud, select your PC files).

Author: EAA_00