The Wall Of Flesh (WOF) is the hardest Pre-Hardmode bosses. Killing him drops the Pwnhammer, which initiates Hardmode. He is summoned by using a Guide Voodoo Doll in Hell (The Underworld). To find one kill a Voodoo Demon, and have a Guide in your world.
Step 1: Preparation
You will need lots of items to beat the WOF. I used:
-Cloud In A Bottle
-Band Of Regeneration
-500+ Silver bullets
-Lots of Hellfire arrows or Jester arrows
400-600 Blocks (Can be any kind)
Step 2: Making the Arena
Make a bridge from the beginning of the Underworld to near the end. This will let you walk across and save time. Mine any obstacles that are in your way.
Step 3: The Fight
Keep your distance and hit the eyes, they have no defense. Use Muramasa and the fire flower to kill the hungry. If you start to loose health run away, you have the entire underworld to defeat him. Once you get good at beating him you will be able to farm him for about 20-30 gold coins each. Hope this was helpful for you, KIK me if you need further help. http://terraria.gamepedia.com/Guide:Wall_of_Flesh_strategies <--also a good source for information.