I will assume you are going for this after completing all other achievements and have exceptional gear and all NPCs. See my other solutions for hints on crafting stations, getting NPCs, slaying bosses, etc. I will also assume you know how to duplicate (if not see below).
The achievement requires you cover 50% of the world in Hallow/Corrupt blocks. Mud, Snow, Ash, Open Sky, etc. cannot be turned Hallow/Corrupt. The only blocks that can be Hallow/Corrupt are Dirt, Stone, and Sand!
The Hallow and Corruption spread on their own in Hard Mode. Corruption moves slightly faster. This is the fastest and most efficient way to get this achievement.DUPLICATING METHOD
HALLOW or CORRUPT
1. Start a new Small world if you do not have one or be prepared to terraform your current world. Grab some friends to help or be prepared to spend many hours working on this.
2. Build a housing structure in the sky close to the initial spawn area. I suggest using bricks and make the outer walls two bricks thick. Kill any boss to get the Dryad to move in. You will need her to check on your progress.
3. From this, build a Hellevator and a Skyvator. This is easiest by duplicating Dynamite or using Rocket IV from a Rocket Launcher to blow out a path straight down.
4. Fall down the Hellevator and kill The Wall of Flesh to unlock Hard Mode. (Optional)Then kill a Mechanical boss to get the Steampunker to spawn so you can buy clentaminators and solution.
5. Get a clentaminator and lots of solution (Purple for Corruption and Blue for Hallow).
6. Drop down the Hellevator and spray to the left.
7. Drop down the Hellevator and spray to the right.
8a. Make other shafts using the same method as the Hellevator and repeat 6 & 7 to spread the Hallow/Corruption all over under the ground. Check with the Dryad when you are finished to see how much percentage the world is Corrupt/Hallow
8b. You can dig across horizontally from the Hellevator and spray up and down, but this takes much longer.
NOTE: At this point, you can idle boost and let the Hallow/Corruption spread as far as it can reach, but bear in mind that Corruption cannot spread to Hallow or Crimson and Hallow cannot spread to Corruption or Crimson. It is also unlikely that enough will spread to get the world to 50%.
9. Go up to the top of the world using your Skyvator and build a brick bridge across the world. Build a similar one out from the housing structure.
10. Duplicate Pearlsand for Hallow or Ebonsand for Corruption and fill lots of chests.
11. Drop sand from the bridge to fill in the area between the bridge across the top of the world and the lower bridge you built. Keep in mind that NPCs cannot live in the Corruption, so for Corrupting, you will want to place the sand far enough away from the building to not affect them. Pearlsand can be placed right next to or all around your NPC structure. Sand is recommended because it falls when there is nothing under it and you can hold down
to place it continuously.MNRussell said:
Try to purchase if possible the Brick Layer accessory from the Travelling Salesman. It doubles the speed at which you drop sand.
12. Continue checking in with the Dryad until you get to 50% Corruption/Hallow.
This is the fastest and most efficient way to get this achievement.
If you have any notes, let me know and I will add them with credit before you vote negatively.