I have beaten the Wall of Flesh but can't get to the loot, as I died from the lava just after. The Hardmode enemies are too difficult to kill and do so much damage that I can't get past them to reach the Underworld. Does anyone have any tips or ideas to help me get the loot? Your help would be greatly appreciated.
While the Pwnhammer is instrumental in getting easy access to hardmode ores, there is another way!
Crates can be fished from the water! These crates have a chance to contain hardmore ores, allowing you access to those more powerful armor and weapon pieces. Depending on if you're playing the PC, Console, or Mobile versions, your access to certain crates may be denied, as the Console and Mobile versions only have Wood, Iron, and Golden crates.
Better bait and better fishing rods will give you greater chances at obtaining crates from your fishing excursions. The Angler's quests will give you access to better rods and bait over time, but these can take some time to complete. Once you're better geared up, you can make your way down to the Underworld to get your WoF loot, and maybe even fight it a few more times.
The traditional way to reach the Underworld level, post-Wall Of Flesh, is to use a Hellevator: basically a straight-vertical two-block hole in the ground, leading all the way from your home base down to the Underworld. You can buy a bunch of rope from the Merchant to let you go up and down easily. This lets you avoid monsters because (1) you're moving past them very quickly, and (2) most of them can't get at you through walls.
Usually you build your Hellevator before starting Hardmode, but you can probably get it to work afterward if you're careful. Make sure to line the sides of your Hellevator with blocks -- you don't want monsters wandering in! There are various ways to deal with pools of water and lava in your path, but my favorite is to drop bombs until it breaks through and drains the water/lava into a cavern somewhere below.
Burrowing and phasing enemies will be a problem for you, especially if you get too close to the Hallow or the Evil biomes. All I can suggest is to Magic Mirror out and come back when they despawn.
Once you get to the Underworld, things will get easier -- no Hardmode enemies spawn naturally in the Underworld at the start of Hardmode.
From the comments in Kaizerwolf's answer, I like to offer an alternative to this situation. While a pwnhamer is conveniently dropped by WOF and can destroy the Altars to spawn and increase the amount of the three hardmode initial tier ores, it is not the only means forward.
As per Kaizerwolf's answer, you can obtain the ores and even the bars of the hardmode ores by fishing, I instead suggest that you create a Mythril/Orihalcum Anvil also a Titanium/Adamantium drill/pickaxe, during which you may have enough Cobalt/Palladium for some weapon or armour pieces for going to hell(to retrieve the pwnhammer). This will enable you access to all three tiers of hardmode ores before you go smashing Altars and harvest the larger amount required for weapons and armours.
I'll also introduce a few items that'll make this method more efficient:
Crate Potion as it's name suggest, increase chance of obtaining crates from fishing. I believe it doubles the base chance from 10% to 20%
Sonar Potion less effective than crate potion but this allows you to reject junk and common fishes so you waste less baits on those.
Fishing Potion downright increases fishing power which in turns slightly increases chance to get crates.
Angler Tackle Bag high tier crates(gold and biomes) get increased chance of appearing when fishing power is high, this accessory will boost fishing power while also letting you use/waste less bait while fishing, it can be obtain even before hardmode but also while you are fishing for your ores, just make sure to do the Angler's quest daily.
Buggy/Enchanted Nightcrawler These two are baits that are not too hard to obtain while having decent fishing power. Buggy can be found in the jungle which is not too dangerous right into hardmode, just break the bushes and one might appear. Enchanted Nightcrawler is just an upgraded Worm by combining it with a Star dropped at night.
For the actual process of reaching the pwnhammer, I have a few tips:
If that is still not enough use Invisibility Potion, while under its effect, ranged monsters will not shoot at you. And seriously, if dodging slimes and bats are too hard I have no more solutions to offer.
Additional option from Tom's comment(also mentioned by JonK in comments of the question) is to create another world to fight the WOF in pre hardmode conditions again, this time not making the same mistake.
And if out of world resources is not a taboo for you there are also world editors available around that you can use to reset the world into pre hardmode for a retry.
Just take note that if you are playing in your own world once you leave the items will be gone so when you reach that spot again after returning the Demonite or Crimtane bricks will be left.
If you have any teleportation potions, you may be able to use them and get lucky. It's possible to fish up Chaos Fish from the Underground Hallow to make more, but if you aren't able to get there safely, then a hellevator is a good idea. If your usual route to the Underworld is reasonably easy and vertical, applying thorns potions and inferno potions will allow you to damage anything coming near you, as will accessories such as the Honey Comb. If you swing a sword the whole way, with some of the potions, you might just make it, as long as you bring a few healing potions.