Elixir Counter is an android app that gets you your opponent's deck as soon as you start a battle, then you can manually click on the card your opponent uses to keep track of their elixir & card rotation.
Elixir counting tutorial (enable elixir counter mode in settings):
Step by step:
1- Press start and grant permissions to the app.
2- Press "Open Clash Royale" button.
3- Start a battle then press "Start Elixir Counter" button as soon as you see the loading screen.
4- Wait a few seconds till you see your opponent's deck.