Package downgrade from live environment

I run arch Linux on an hp laptop. I installed a graphical login manager (gdm) and during boot, the kernel unpacks, systemd starts, but when it attempts to start my login manager

"An error has occurred :(. Log out and try again.

[Log out]"

But I can't select that button. I know that uninstalling it will repair my system, or at the very least, stop the bleeding. I need to be able to boot into a thumb drive, access my installation and sudo pacman --remove gdm BUT, I am not sure the exact procedure to do so without messing up my system. What partitions do I mount, do I need sudo, do I need to tamper with any init scripts? The laptop is fairly junk, and nothing important is on it, but I don't want to reinstall for a misconfigured package.

EDIT: Ok, I have found that with the arch-chroot scripts, I can get into an install by mounting all partitions mount /mnt /dev/sda1 /dev/sda6 ; arch-chroot (something, I think the drives I mounted(This is most of the part I need help with))

archiso@joeyhobbypcarch / # su averagejoey2000
averagejoey2000@joeyhobbypcarch / $ sudo pacman -R gdm fprintd ; yaourt -R (package 1 2 3)

Is any part of this wrong? I know the partition table by heart, I can find the buggy package within minutes of controlling the device.

EDIT: Added caveats! Screen is mostly busted. How quickly can I have it output to an external monitor? I don't have an arch iso handy. Can I chroot from an unrelated live os; say "T.A.I.L.S."?

Boot Live OS Terminal

su
mkdir /mnt/arch
mount /dev/sda1 /mnt/arch
mount /dev/sa3 /mnt/arch/boot/
mount /dev/sda6 /mnt/arch/home/

Using chroot Mount the temporary api filesystems:

cd /mnt/arch
mount -t proc proc proc/
mount --rbind /sys sys/
mount --rbind /dev dev/
mount --rbind /run run/
cp /etc/resolv.conf etc/resolv.conf
chroot /mnt/arch /bin/bash
su #sudoer
pacman -Q |grep gdm|sudo pacman -R - && yaourt -Q|grep logo|yaourt -R -
exit
telinit 6 #Remember, we were root during the file mounting. No need to do it again.