Connect 12V, serial console and networking.

Optional: Update APU2 BIOS.

Download the latest ISO image from https://www.archlinux.org/download/.

Verify the SHA sum.

Copy image to USB.

Boot the APU2 and press F10, select USB boot.

Switch console to 38400 baud. Press 'ctrl-l (lima)' to redraw the screen.

Select the Boot Arch Linux option and press TAB.

Add console=ttyS0,38400 to the kernel line and press enter

Log in with user root (no password).

If you connected the network cable after booting, request an IP-address

# dhclient enp1s0

Install and run SSHd to complete the installation over SSH:

select nearby mirror in: /etc/pacman.d/mirrorlist

# pacman -Sy
# pacman -S openssh
# passwd root
# systemctl start sshd

https://wiki.archlinux.org/index.php/installation_guide

Configure serial console, when still in chroot and just before the reboot.

# /etc/default/grub   # add options below
GRUB_CMDLINE_LINUX="console=tty0 console=ttyS0,115200n8"

Configure grub and serial:

# /etc/default/grub   # add options below

## Serial console
GRUB_TERMINAL_output=serial
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"

Make new grub config:

# grub-mkconfig -o /boot/grub/grub.cfg

Reboot and connect with 115200 baud.