I used Fly mode only to create this guide. I have already completed it legit so I wanted to make this guide to help others, and using it helped in getting a few screenshots which showed certain parts of the underground trials. Luckily you don't have to do the above ground trials just to open the door to this one, due to the convenient checkpoint.

Remember that you lose all horizontal momentum once you hit a jump pad. If this guide helped you, leave a comment saying how many tries it took you to complete!

Load the "Too Old For This ♥♥♥♥" checkpoint in E2M2, you will spawn in front of the open middle door.

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Go inside and turn around, look on the right side on the floor for a switch.

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Step on the switch to open the door.

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Head out of the room, turn 90 degrees counterclockwise and run off the ledge. Turn around and you'll see the open door.

Head inside and to the right to find a hole in the ground.

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Drop down to begin the first trial: the lava pit.

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Continue bouncing over the spikes.

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...and eventually getting over the next fire pit while dodging the fireballs.

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FALL DOWN THE LEFTMOST HOLE. The other two will kill you.

You will instantly be greeted by a fast-moving lava wall of death.

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If you are facing it turn around and head to the jump pad. If you are not facing it, just head to the jumppad, either way use the jumppad to bounce up to the next room.

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You will see a series of many flame pillars spouting in this section.

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Carefully dodge them to successfully come to what you came here for: Scott's Mystical Head.

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Take the now active platform to the surface.