Terraria – Paladin and truffle won’t spawn

You can go exploring some corrupted/crimson areas (randomly picked at world generation) to destroy the shadow orbs/crimson heart waiting for you at the bottom of chasms.

Each shadow orb/crimson heart destroyed will provide you some loot, and make a meteorite land on the world. Once the meteorite has fallen, go find the site, and mine the meteorite ore. This will allow you to craft some meteorite gear and weapons.

Once you've destroyed three shadow orbs/crimson heart, it will make the Eater of Worlds/Brain of Cthulhu spawn, allowing you to fight two different bosses.

Note: The shadow orbs are protected behind some ebonstone/crimstone, which you will not be able to mine with your current pickaxe. Just buy some dynamite from the Demolitionist and use it to access the shadow orb. Or you can buy purification powder from the Dryad, which turns the ebonstone/crimstone into the easily destroyable standard stone.

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.