I was searching for a Lost Girl/Nymph for like 4h or something and started loosing my patience
so I started collecting some information about that mob. I watched some Youtube-Videos and checked some Forum threads to find how long it will take to search for "her", some people found two in the first 30 minutes, others needed about 20 hours and some people were searching for 200 hours without finding her. So its was not an option to search for her in a normal way. I started to search for Nymph farms but there was not a really helpfull video or a guide.

If this guide helped you to get a metal detector or the achievement I would like to know in the comments and appreciate if you rate this guide.

Dont send me friend requests! I haven't played in ages so I can't gift you items anymore sorry :(

This guide is from 2015! (V1.3 I guess) I try to update all the information but I sure I will slip a few things please have this in mind.

During the research I found out that one guy checked the code. Turns out that the spawn rate in pre-hardmode is about 0.6%, in Hardmode its only 0.1%. The reason for this is that the spawn rate of the Nymph increases, but the varity of mobs is much greater, so technically the absolute appearance chance of the Nymph decreases. So basically, you just have to force as many monster spawns as possible to increase the chance of getting a Nymph. A lot of information about the spawning algorithms you can get from HERE . Its a video from ZeroGravitas which shows a truffle worm farm.

To set up the farm it will take a bit of time because you have to prepare a great area.
As you can see, at the moment of creating this guide it's not completely finished, but it works already

So lets start, what can we see here ? Lets say its called the 'core' of the farm.
You can see a platform at the players position.

80 140 blocks of mud with jungle grass which increases the spawn rate by 150% (in this picture of course it's not fully grown).
Thanks to Ralle(hop) for the hint.

On the platforms were the player is located in this picture is also a water candle which increases the spawn rate by another 25%.

To the left of the platform is one rock block with a pressure plate (too lazy to buy a switch which is much better in this situtation) which activates the safe zones.

You can see a few banners from the monsters which get killed in the picture above.

• your farm should be in the 'underground' or at 'cavern' high
  
• you have to prepare a lot of space (red) which means you dont have to dig it out like I did here
• you have to prevent any spawns in this area (as you can see in the left corner there is not everything digged out, but it's no space to spawn) so you could also fill up the whole space so that nothing can spawn
  
• there should be no space bigger then 2x2 blocks.The spawning area (orange) has to be 35+ blocks below your platform, they wouldnt spawn if it's less then this.
  
• I didnt build it in the center, instead a bit more to the left so the mobs would try to attack the player and fall on the lava floor.

The lava floor has two tasks : primarily to stop monster spawns on the ground, secondarily to kills mobs without destroying their drops.

Activate your circle to activate the safe zone (blue) to safely loot your drops.
Safe zone means anykind of solid block which is triggered through a wire to appear/disappear.

From now on you just have to wait until a lot of monsters died and check the drops.

In my case I got two metal detectors in an hour, but maybe I just had a lot of luck, so write how many you got or if you even got one at all.

So you killed a lot of monsters with this method, but didnt do any damage, which means if a Nymph died you will get nothing except her loot.
You need at least one minion (stardust dragon) which goes down against some monsters from time to time and deals a bit damage.
If the Nymph gets hit one time and dies in the lavafloor you will unlock the achievement.