The Wall of Flesh is the last boss you need to defeat to have your world enter Hardmode. It's a monumental step towards taking on the final boss, Moon Lord. The Underworld contains the next strongest ore you'll need to be able to survive Hardmode, but the peace of mining it can only be accomplished by defeating the Wall itself. Although you could mine it and use it before taking on this boss for ultimate protection.
There are various different ways to go about killing bosses, from frantically swinging and hoping for the best to strategic use of tools and platforms. If you've been having a tough time figuring out which works for you, maybe this guide will hit the spot. It's always best to prepare a good amount before such a big excursion.
4 Armor
You'll need the best armor a man can get before going to the depths of Hell itself. In Pre-Hardmode the best armor underneath Molten, which is acquired from the Underworld, is Platinum, Necro, Shadow, or Crimson armor. Your world will only either spawn Gold or Platinum ore, so if you have Platinum it's best to mine for that. Necro armor is crafted by combining cobwebs from caves and bones from killing Dungeon skeletons. Shadow and Crimson armor are from the "evil" biomes in your world. You'll need a Gold or Platinum pickaxe to mine the Corrupted or Crimson ore known as Demonite and Crimtane. This is an excursion in and of itself, but if you're not ready for the evil biome, you're not ready for the Wall of Flesh!
3 Weapons
Next on the list for preparation is the best weapons you can find. This would be either swords Night's Edge, Fiery Greatsword, or Bee Keeper. There are up to 49 Pre-Hardmode swords and higher than 50 for other types of weapons. Any and all will do in the grand scheme of things so long as their damage is 28 and higher. You'll want something that's both fast and that has projectiles. This way the Wall closing in on you won't be as much of a problem because you'd have projectiles from a distance and once the Wall gets close you can hit fast to do more damage.
The Bee Keeper expels little bees that do extra damage and is found as a Queen Bee drop. The Fiery Greatsword lights your enemies on fire and is crafted with Hellstone if you rather mine that before taking on the Wall of Flesh. Night's Edge is the ultimate Pre-Hardmode sword that is crafted by combining four other strong swords at a Demon or Crimson altar. You need the Fiery Greatsword, Muramasa, Blade of Grass, and either the Lights Bane or Blood Butcherer which depends on your evil biome.
Fast and strong yoyo's like Valor from gold chests in the Dungeon or Cascade as an Underworld enemy drop after Skeletron is dead are good here. So are flails like the Sunfury which are found in Underworld shadow chests, and boomerangs like Thorn Chakram which is crafted with Jungle Spores and Stingers.
If you're going the magic route, Magic Missile, Crimson Rod, and Flower of Fire might be for you. The Magic Missile will hone in on enemies and find them for you if you don't directly click one and is found in golden chests in the Dungeon. The Crimson Rod spawns a rainy cloud that does rain damage. With this you can just place it above enemies and let it rain. It's found from breaking Crimson Hearts if you have the Crimson biome and from Crimson Crates when fishing. The next best thing is the Flower of Fire which can be looted from Underworld chests and lights enemies on fire. Don't be afraid to switch between more than one weapon when in battle!
2 Tools
If you never want to return to the Underworld again, best to bring the best pickaxe you can so you can mine that Hellstone right away. Hellstone is the next best ore you'll need to survive your world turning into Hardmode. Platinum or Gold pickaxes will do. Summoning staffs are also highly useful. They can be coined as weapons but they are definitely tools of destruction.
Bringing a summoning item with you for the fight ensures damage is always being dealt and protection is always there. If you're panicking or switching weapons, your summon will continue doing damage for you. The best Pre-Hardmode summoning tools to use are the Imp Staff, Hornet Staff, Ballista Staff, and Explosive Trap Staff. The Imp Staff is crafted with Hellstone if you want to mine it before fighting. The Hornet Staff is crafted with Bee Wax dropped from Queen Bee. The Ballista and Explosive Trap summons are sentry turrets that stay in one place. These will be more difficult to use as once the Wall of Flesh passes by them they'll be useless. You would want to keep replacing them at moments notice or only use them for beginner damage. With the Ballista, you'll want to place it behind you. With the Explosive Trap you'll want to place it where the Wall is about to move.
1 Blocks
Last but not least, blocks. The Underworld is just pits of lava with some small apartments here and there. There's no where to walk and be safe. You'll need to set up majority of the Underworld for safety and for running before starting the boss fight. Take a summon with you so you don't have to be annoyed with all the small mobs that pass by and make sure to make a path or bridge almost completing the entirety of the Underworld.
You probably have around 399 blocks of some random block type from all your adventures, so use that and keep building to the left and right of the map until you think it's enough running space to complete the boss. Remember that the Wall of Flesh will keep coming toward you until it's dead, not moving or turning around, so you just have to keep running left until it's dead. You'll want to make sure you don't accidentally run to the end of your bridge and fall into lava. Make it long!