How to survive an immortal imp army?

EDIT: Since version 1.0.5, Fire Imps aren't really a problem anymore. One needs only create some sort of shelter (for, instance digging a mine shaft with a roof) to shut out the demons and lava slimes. Bone Serpents and Fire Imps rarely spawn anymore and when they do, in much smaller numbers.

So it seems my tunnel won't get any deeper. I've reached a point where imps are spawning like rabbits. I mean this is just stupid. Any advice on surviving this crap?

I wrote "immortal" in the title because all the imps are behind stone walls and launching volleys of fireballs at me while I'm stuck in a narrow mine shaft.

You can see I did my best with my bow at the two imps in the lower left cave… but things went south when two turned into five

Solution 1:

Fire Imps spawn where there is enough room. You can build a grid with stone and make sure the holes in the grid are just small enough, so the Imps wont spawn there. That is how I do it.

Also, with advantage, you can use melee weapons to deflect the fireballs coming from the Fire Imps. Higher attack speed will of course make it easier to hit the fireballs.

For more information on Fire Imps, take a look at the Terraria Wiki.

Where it also states:

Fire Imps cannot spawn in tunnels three blocks high by two blocks wide.

So building your grid around that information should work. And thus keep Fire Imps from spawning.

Solution 2:

Here's an interesting idea:

Bring 50 blocks of meteor ore with you when you venture into The Underworld, and place them in large chunk of contiguous blocks. This will create an artificial "Meteor Biome" identical to that which is created when you get the "A Meteor has landed!" message.

When the player is in a meteor biome, the normal enemy spawns are overriden to spawn only meteor heads instead. This means that instead of teleporting, fire-lobbing fire imps and screen-wide chasing bone serpents, you'll only have to deal with the relatively predictable meteor heads instead.

Just don't forget to move your artificial meteor along with you as you mine your way across the Underworld.