Elden Ring: Shadow of the Erdtree currently has a 'Mixed' status on Steam, with many of the negative reviews complaining that the bosses are too hard [also game's PC performance]
My only issue is that since the DLC was released I can no longer play online on linux. It says "Inappropriate behavior detected" no matter what. Didn't even buy the DLC.
Is your proton set to experimental? because mine is working on arch. If it is, try verifying files, or if that doesn't work, back up your saves and nuke the prefix.
To add to what everyone else said, it's a known issue. Easy Anti-Cheat isn't working properly on Linux unless you own the DLC. It's working fine on my end since I own it.
That's the issue, game works fine with the DLC and fails without the DLC.
At least when playing offline you can use er-patcher to play it without framerate limit, chromatic aberration and in ultrawide. https://github.com/gurrgur/er-patcher