1. Choose one version of Proton for playthrough and don't change it afterwards to not increase chances of transition bug or *maybe* something else. You may try GE-Proton, which should work better.
*Again, don't change proton version mid playthrough or at least backup your saves before that. It's not guaranteed to give you any problems, just good practice.
2. Download protontricks. In order to game run smoothly and draw text properly run these commands to install needed packages:
protontricks 250520 dotnet452
protontricks 250520 xna40
*If you have troubles with using protontricks, you can always install these packages directly in steam underrail prefix via winetricks. Research how to do it on the net.
**From this point all is set, you can start playing.
3. IF you have trouble with mouse, try adding this in steam command line: DXVK_ASYNC=1
4. IF you have unplayable stutters after some playtime add this to your steam command line: LD_PRELOAD=""
*I don't know what it is, i don't want to know. *It also fixes this problem across all games running via proton!
5. IF your .png custon portrait renders funny in game, like not transparent background, try compressing .png file one or more times via whatever tool you see fit.
-MAKE SURE that in game settings option "Sprite Target Mask" is CHEKED. or you gonna have a bad time

-Try disabling vsync to see, maybe game will run better. Who uses it anyway.