The Wall of Flesh is generally considered the hardest Pre-Hardmode boss, so equip powerful items. Some of these may include:

The Shangara
The Phoenix Blaster
Flamarang
Fiery Greatsword
Beenades
Phasesaber
Night's Edge
Dark Lance
Sunfury
Demon Scythe
Flamelash
Crimson Rod
Molten Armor
Obsidian Shield
Cloud in a Bottle
Water Walking Potion

, You can open your map, and if you see the Guide's icon, he's still alive.

If your Guide isn't alive, respawn him.
 You can do this by continuing downwards in the Underground Jungle until you find a hellish looking place., Tap the bed.,

At random intervals, place down Chests with Potions and Ammo in them.
 You will need to cast this into the fire to summon the Wall of Flesh.

Sometimes, the Guide Voodoo Doll falls into the lava after a Voodoo Demon is killed, unintentionally spawning the Wall of Flesh. Don't panic. Just begin the battle.
 It can:

Shoot lasers
Spawn minions known as the Hungry.
Spit leeches that drop Hearts upon death.
 Don't run too far away from the Wall of Flesh, as it will pull you back and possibly kill you.,, After they're defeated, you can shoot the eyes of the Wall of Flesh to kill it.
Prepare for the battle.
Make sure your Guide NPC is alive.
Travel to the Underworld.
When you get to the Underworld, place a bed inside a Ruined House or any other secluded area.
Using durable blocks, make a bridge that spans about halfway across Hell.
Kill a Voodoo Demon to get the Guide Voodoo Doll.
Learn the Wall of Flesh's moves.
Begin running away from the Wall of Flesh.Kill the Hungry with Beenades.
Defeat the Hungry.
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