partial upgrades on distros without hard linked dependencies is a disaster caused by the user.
You should never have a system with less than 20% free space, but I mean system, not /home, not /var/cache/ of /var/cache/pacman,
Make partitions and mount things separately, especially /home
In a pinch you can live without man-pages remove /usr/share/{doc,man,html}/*
and on /usr/share/locale/* keep just the ones you use