Terraria

You will get this through natural gameplay as you make items at your anvil(s), such as armors, weapons, candelabras, etc. It takes 300 items crafted at any anvil to get this achievement, not 300 bars used at an anvil. So 1 piece of armor counts as 1 item, regardless of how many bars it takes to make it. Terraria saves things to your profile as you do them, such as boss kills, items crafted, etc. whether you save the game or not using Auto Save or manually saving. So it does not matter if you do this in your own world or another person's world and it is cumulative.

QUICK SOLUTION
Duplicate or save up either 300 Iron bars or Chlorophyte bars, then go to an anvil and start crafting:
Iron Bars - 1 Iron bar makes 10 Iron Chains at ANY anvil
Chlorophyte Bars - 1 Chlorophyte bar makes 50 chlorophyte arrows at Mythril or Orichalum anvil

NOTE: 1 piece of armor, 1 weapon, one set of 10 Chains or one set of 50 chlorophyte arrows counts as 1 item crafted.

You can also craft iron fences from iron bars then convert them back to iron bars at an anvil to make progress as well, essentially converting them back and forth until you get the achievement (confirmed by Lanceride). To do this, you can go to your crafting menu with Iron (or Lead) Bar in your Inventory and stand an any anvil. Then press so you can only craft what is available and go to Iron/Lead Fence under the pickaxe and hammer. You can then hold down until the achievement pops as you will make iron/lead fences and then back to iron/lead bars over and over.

Alternatively, just like "I'm Smelting," you can craft the items with Auto Save turned off and reload the world after making items over and over to get the achievement if in single player and wanting to boost it.

DUPLICATING METHOD


If you have input, let me know instead of negative voting. I am happy to give credit.

I found the easiest method in order to get this much easier than duping!!
All you need is 1 IRON BAR then go to a anvil make sure all you have is an iron bar in your inventory. Now set crafting to only things you have available not show all. Now make a metal fence this will give you one craft. This is where the trick comes in you can now turn your iron fence into an iron bar.

So literally hold your finger and it will fast craft between bar and fence and will give you this achievement in seconds with one item!!