I can confirm this to be working flawlessly, I had to use PROTON_USE_WINED3D11 to fix some of the visual artifacts, but that's just me running this on 390.x nvidia drivers.

With proton 4.2-3 is working nice and it can be played from start to finish, maybe with some frame drops here and there.
Level 3, 4 and 5 do not load correctly when you come from the previous level, having to restart the game to load them properly.
Full system report