First, go to your settings. set your difficulty to whatever, and set "minimal hud" to on.

After you set it to minimal hud, you want to enter [initial], and walk by the gun. DO NOT PICK IT UP, OR IT WILL NOT WORK. Walk and enter the portal. It is REQUIRED to have tutorial uncompleted. If it isn't, then you'll see this screen.

Click 13 times on play. No more, No less. After which, you'll be on [Final], with the Pacifist ending achieved. Happy Speedruns!