The default included Arch ISO menuentry does not work.

[coolymike]

It simply says error: file '/arch/boot/x86_64/archiso.img' not found. when trying to boot.

[adi1090x]

Try this one:

menuentry 'Arch Linux ISO' --class arch {
  set root='(hd0,YOUR_PARTITION)'
  set isofile='/PATH/TO/ISO'
  set imgdevpath='/dev/disk/by-uuid/UUID
  loopback loop $isofile
  linux (loop)/arch/boot/x86_64/vmlinuz-linux archisodevice=/dev/loop0 img_dev=$imgdevpath img_loop=$isofile earlymodules=loop quiet splash vt.global_cursor_default=0 loglevel=2 rd.systemd.show_status=false rd.udev.log-priority=3 sysrq_always_enabled=1 cow_spacesize=1G
  initrd (loop)/arch/boot/amd-ucode.img (loop)/arch/boot/intel-ucode.img (loop)/arch/boot/x86_64/archiso.img
}

[V8AA]

This one worked for me

menuentry "Arch Linux ISO" --class arch --class archlinux --class linux {
    set root='(hd0,1)'
    set isofile="/PATH/TO/ISO"
    set dri="free"
    search --no-floppy -f --set=root $isofile
    probe -u $root --set=abc
    set pqr="/dev/disk/by-uuid/$abc"
    loopback loop $isofile
    linux (loop)/arch/boot/x86_64/vmlinuz-linux img_dev=$pqr img_loop=$isofile earlymodules=loop driver=$dri quiet splash vt.global_cursor_default=0 loglevel=2 rd.systemd.show_status=false rd.udev.log-priority=3 sysrq_always_enabled=1 cow_spacesize=1G
    initrd (loop)/arch/boot/intel-ucode.img (loop)/arch/boot/amd-ucode.img (loop)/arch/boot/x86_64/initramfs-linux.img
}

[coolymike]

Solved by V8A-yes