The minute I reach the border of the underworld, fire imps start popping up everywhere, nuking me with fireballs from every conceivable angle. In addition, 3 huge skeleton worms show up ramming into me relentlessly.
The end result is that I'm hit by something at least a couple of times per second, knocking me around. Mining is impossible as I'm knocked into the lava if I even get close, and building some sort of fortress won't help seeing as all the monsters down here ignore walls completely.
I have 300 hp, 200 mana, full shadow armor and a nightmare pickaxe. How the hell do people survive down here?
Best Answer
Find a place where below you there's a lot of ground (ash) and hellstone (no buildings or anything where the imps can spawn on). Dig a tunnel (width 2) under you (block fireballs with a fast pickaxe) and mine until there are no visible places to spawn for imps, other than your own tunnel (which you can even fill with dirt).
Then it's easy to mine hellstone while blocking the fireballs of the few imps with your pickaxe (always face the enemy while mining). There won't be many imps since they don't have much space to spawn.
A stack of potions, magic mirror and any flail (for worms) should then be enough.
(I think the tunnel in the image is too wide, but hopefully this still makes it more clear)