After spending some time testing out Proton, I've noticed that there are games, not officially supported by Proton yet, that don't function properly without some required wine packages. particularlly Starpoint Gemini 2 (appid 236150) and Sacred 2 (appid 225640). In the case of these two games, they require xact and physx to be installed via winetricks. The solution is simple, but you have to know where you're games are stored. On Arch Linux, the location is default to ~/.steam/steam/compatdata/APPID/pfx/. To use winetricks, just set your wineprefix to the desired appid folder and run winetricks.

$ WINEPREFIX=~/LOCATION winetricks xact physx

This process works with all of the Windows-based games that Proton creates a profile for. Simply find the appid and you can run winetricks to install packages that are required for play.