There is likely a better way to do this, but this is how I auto mount my Nexus devices in Arch Linux.
git clone [email protected]:bugthing/arch-nexus-mtp-automount.git
makepkg -s
sudo pacman -U android-automount-0.<TAB>
cp 99-android-mtp.rules /etc/udev/rules.d/
cp android-mtp.service
sudo systemctl start android-mtp
sudo systemctl enable android-mtp