Arch root partition is full, how to clean it up? - eviltoast

After installing pacman packages (last one was ‘ungoogled-chromium’) my root partition of 20GB is completly full. Now I can’t update new packages.

My partition structure is: root (20GB) /home (470GB) swap (10GB)

How can I delete the garbage that is piling up in my root, and how to prevent it from happening again.