Clone Repo and open with Xcode,
Press:
CMD +R
That will built and run the application. For the app to work you need to activate the REST API based on this repo: https://github.com/DamageCalculator-LeagueofLegends/DamageCalcBackendTS , instruction how to run the server are in that repo.
To calculate the damage first choose champions and items then press the button at the very top right of the screen.