Even tho this might be a bit too obvious for experienced players, it could be useful to new players.
This method provides infinite amount of surtling cores and coal.
1. Find a swamp.
2. Explore it and locate at least one surtling spawner. (Cone of fire coming from beneath the surface)
3. Dig around it so water can get in.
4. Go do other stuff and come back in a while or just hop on a tiny tree platform and wait.
Method suggested by : Dr.VV01F