Disclaimer:
This information below is solely used for my purpose and may not be suitable for others.

1. GRUB2 Default Screen

Use arrow key to navigate the GRUB menu and press e to edit.

2. Add Runlevel Parameter

Scroll down to the line that start with linux followed by the kernel image name. Press the End key to go to the end of line, then hit the space key and runlevel number.

In the image below, single user mode is entered.

• 0 - Halt
• 1 - Single-user mode
• 2 - Multi-user mode (not used for Arch Linux)
• 3 - Multi-user mode with Networking
• 4 - Not used
• 5 - Multi-user mode with X11
• 6 - Reboot

Boot to Selected Runlevel

Enter Ctrl+x or F10 to boot and you have booted into selected runlevel.

That's all!
-gibb