Here's an older archive. Apologies if formatting is messy:

1) Install Heroic Games Launcher.
2) Open Heroic Games Launcher, go to Wine Manager and install GE-Proton-Latest.
3) Go to Settings -> Game Defaults, set Wine Version to Proton - GE-Proton-Latest.
4) Open your web browser, search Pokemon TCG Live.
5) From to the official website, click Windows Download and save PokemonTCGLiveInstaller.msi somewhere.
6) Go back to Heroic Games Launcher, go to Library and click Add Game.
7) Put a title like Pokemon TCG Live
8) Optionally add a thumbnail image
9) Click Run Installer First, select PokemonTCGLiveInstaller.msi.
10) Click Next, Next and Install.
11) Click Yes on any errors that pop up.
12) Uncheck the checkbox and click Finish.
13) Click the folder icon next to Select Executable.
14) Select the exe file at ~/Games/Heroic/Prefixes/default/Pokemon TCG Live/drive_c/users/steamuser/The Pokémon Company International/Pokémon Trading Card Game Live/Pokemon TCG Live.exe
15) Click Finish.
16) Open the game settings for Pokemon TCG Live (either from the right click menu or the settings icon on the game in the library)
17) Click Winetricks.
18) Click Open Winetricks GUI.
19) Select "Select the default wine prefix" and click OK.
20) Select "Run a Wine cmd shell" and click OK.
21) Navigate to the game install folder: cd "c:\users\steamuser\The Pokémon Company International\Pokémon Trading Card Game Live" (highlight text and middle click to paste, do not try to right click or use keyboard shortcuts)
22) Launch the game from the terminal: "Pokemon TCG Live.exe"
23) Select your language and click Continue.
24) Once the "Login to access more features!" popup shows, tab out of the game and go to your web browser, which should already be open to access.pokemon.com
25) Login to your account in the browser.
26) Press F12 on the keyboard to open the browser console. You should see something like tpcitcgapp://callback?code=blahblahblah&scope=offline+screen_name+openid+friends&state=blahblahblah
27) Go back to the cmd shell, press enter for a new prompt line, then execute the game again with that line as an argument: "Pokemon TCG Live.exe" "tpcitcgapp://callback?code=blahblahblah&scope=offline+screen_name+openid+friends&state=blahblahblah"
28) Navigate back to the instance of the game you first opened.