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?