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