Surtling cores are used to make portals to make it easier to travel as well as to build certain items like furnaces to cook metal ores to upgrade your weapons. You need to upgrade your weapons as much as possible before going into the tombs. Make sure your workbench is as fully upgraded as possible (You build additions to the stock workbench that must be placed near it to upgrade it). You should find the items needed to build these upgrades when you pull out your hammer. The first tombs will be hard but once you get a core you can upgrade and build furnaces so you can mine the tin and copper deposits and process them into bronze and make further upgrades to your weapons. Rince and repeat until you are ready to go to the next biome (mountains) and find silver. Each biome will allow you to upgrade stuff further.