Im gonna teach you how to farm Coal and Surtling Core easy and fast!
How to Farm
Step 1 (Search)
Search for the swamp biome its something like this.
Then if you look around, you will see something like this wich is basically surtling spawner.
Step 2 (Dig)
After you find the surtling spawner, you need to dig around it till you reach water.
Somthing like this.
Step 3 (Enjoy!)
Surtlings will spawn and die immediately because of water and they will drop coal,surtling core,and surtling trophy!
This is how the treasure looks like!