I've lost everything and I don't know how to get it back. How can I repair my system all I have is a usb with slax linux. I am freaking out because I had a lot of projects on their that I hadn't pushed to github as well as my configs and rice. Is there any way to repair my system? Can I get a shell from systemd?
It should work, afaik chroot always use the binaries of the system you chrooted, so you will be able to use pacman normally. I don´t remember if chroot will mount the efi partition by default, you can do this before go to chroot (again, I'm have some memory issues but I believe that /dev does not mount as well if you just use chroot, this is why arch have arch-chroot that mounts this kind of stuff but you can mount before so it should work).
Assuming you are using systemd boot on efi partition (that is likelly if you have not changed the installer defaults), what I would do:
On your live CD run
sudo fdisk -lto get what is the efi partition, usually will be /dev/sdb1 since sda will be your usb, you should be able to see something like that.Then you will mount your endevour partition, in your situation should be
sudo mount /dev/sdb2 /mnt/mydiskbut check your fdisk command output.Now you will have to mount the efi partition sudo
mount /dev/sdb1 /mnt/mydisk/efiThen you can use
chroot /mnt/mydisk/and proceed to do a pacman -Syu, this should trigger the post scripts that create the kernel images on the efi partition.