How do you beat the first boss?

You need to farm the dungeon for a little bit to gather enough exp points towards your crafting abilities. You'll be able to mine for iron, coal, rubies and gold from the first dungeon to craft your basics - furnace, anvil, to process your raw materials, but don't craft all of your materials until you need them, like the planks and soft iron. Some buildings just require the raw materials. You'll be able to build an archery lot, a full plate armour statue, weapons rack if you want to be extra secure. I've found that using bows with unique stats of poison damage to help quite a lot with DOT while you're at you're dodging attacks. Remember that the boss has telegraphed attacks that has open windows for your damage. After the second AOE attack, he will jump on your current location, so always be on the move while he jumps!