Go to Options and select Password from the list.
Type in: Y8VM 7HVH BCPZ PJVB
This loads you on level 24 on Be Gentle! with full ammo and weapons.
You can savescum through this. It's tricky but doable.
Hit the switch in front of you and go down any drop.
Use the BFG Ammo on the mobs and first Cyberdemon. Achievement unlocked.