My steps to get SSSiyan trainer to work on Arch linux:

1. Put fluffy mod manager into "steam games path/common/"
2. Add fluffy as a non-steam game
3. Force compatibility with proton
4. Launch fluffy through steam, it should be able to detect your game file as Z:...
5. In the game mod list, go to downloads and get RE Framework Add this to launch options, it works with mods that uses dinput8.dll WINEDLLOVERRIDES='dinput8.dll=n,b' %command%
6. Alternatively, Download Protontricks -> Run Protontricks -> Select "Devil May Cry 5"->Default Wine Prefix->Run Winecfg -> In Winecfg, go to library and add "Dinput8"
7. Check RE Framework in mod list
8. Launch game to test if RE Framework is running
9. Exit the game, Repeat and test SSSiyan Trainer
10. If you crash on title screen, go to your DMC5 game directory, edit dmc5config.ini, change from DX12 to DX11 TargetPlatform=DirectX11