THIS SOLUTION NO LONGER WORKS SINCE IT WAS MADE FOR AN OLDER TUTORIAL WORLD
Many people think this achievement is hard, but it's rather not. Load up the tutorial, and head out the gate into the village. Go to the beacon (the big beam going up into the ski) and take the iron ores placed inside the chest next to the beacon. After you have your iron ores, make an iron sword at a crafting table. Now when you have your iron sword, you need to go to another location. There should be a map in your inventory. Take it out. On the top right corner of the map there is a bunch of numbers, these are coordinates. When you are moving around with the map out, you will see that the numbers change depending on where you are standing. You will need too move in a direction where the coodinates will show: X: -211 Y: 83 Z: 331. It's easier than it sounds. Just take out your map and walk around, and you will soon understand how it works and where to go.
Follow those coordinates until you see a giant ship with a mansion besides. Go to the big ship and follow the wooden path to a ladder. Climb the ladder and jump out at the second balcony/entrance, then walk forward to the end of the corridor and go into the room that's directly opposite of the door where you entered the mansion, if for some reason you can't find it, just go to the exact coordinate I displayed above, it's the coordinates for the room. When you're at the room with a bunch of chests, climb up the ladders to access the second storage room.
Now find the following ingredients: Nether warts, blaze powder and glowstone dust in the chests. After this, take an empty bottle of water at one the chest and fill it up at the cauldron (placed in the same room). When you have filled up the bottle, place it in one of the brewing stand (also located in the same room). Then add the ingredients IN THE FOLLOWING ORDER: Nether Wart - Blaze Powder - Glowstone Dust. If you did it right, you should have a potion of strength level 2. This is a good time to save in case anything goes wrong. In fact, you should save frequently, because you never know what's going to happen. Grab the potion and pull out your sword. Wait until it's night time for the mobs to spawn in. When it's night time, go out and find any hostile mob(zombies and skeletons are the best). Then drink the potion and attack the mob with your iron sword. I think that any mob will work, but I don't know if this is correct, so I wouldnt risk it. If everything went according to plan, you should be an achievement richer :)
EDIT: I have now made big changes to the guide. Many people told me that the guide was hard to understand, especially for newer players, so I have tried to explain things in more detail. Hopefully everyone will understand what to do now :)