Found this info on kernel panic:
Roll back to previous kernel version
If you keep your downloaded pacman packages, you now can easily roll back. If you didn't keep them, you have to find a way to get a previous kernel version on your system now.
Let's suppose you kept the previous versions. We will now install the last working one.
First you need to get the kernel details.
# cd /var/cache/pacman/pkg
# find kernel*
Now use the kernel details in the command below.
# pacman -U /var/cache/pacman/pkg/kernel26-2.6.23.xx-x.pkg.tar.gz
More info here:https://wiki.archlinux.org/index.php/Kernel_panic