Why are you using Archlinux if you need to use QGIS? Isn't this some kind of sickness at this point?
Anyway, there's a nice binary available via pacman, but if you install it you can't use a lot of the tools because of missing Python packages (ugh). You can actually install Python packages via pacman on Arch, I don't know if that's a good idea but that's what I did and it worked.
# Install qgis
pacman -S qgis
# Install required Python packages
pacman -S python-'gdal'
pacman -S python-'psycopg2'
pacman -S python-'owslib'
pacman -S python-'lxml'
And that should do it. If not, just look at the Python error logs from qgis. They should tell you what missing package is currently causing the issue.
