Skype is in multilib repo. Enable it first by editing pacman.conf

# nano /etc/pacman.conf

And uncommenting below lines

[multilib]
Include = /etc/pacman.d/mirrorlist

Then update mirror list.

# pacman -Syy

And install.

# pacman -S skype

More info: https://wiki.archlinux.org/index.php/skype