This achievement is awarded for capturing the Bristly Crake
, a legendary rare bird known to ride the back of Mosswines. I would only recommend going for this after unlocking five-star assignments.
Once you're ready to go for this achievement, start by going to an item box, and equipping the Ghillie Mantle
, which is extremely important for this.
Next, start an expedition to Rotten Vale Southeast Camp (1)
. Once here, talk to your Palico
, and make sure to leave them behind. Then, put on your ghillie mantle, and proceed down the path, jumping down to the left and following along the left wall until you reach a tunnel entrance that has a Mosswine. There is a random chance that the creature we're after will be on the back of this Mosswine.
If it's not, fast travel to Rotten Vale Central Camp (11)
and you'll want to go to Area 15
. On the opposite side of where you enter, you should spot another Mosswine with another possible chance at spawning the rare bird.
If it does spawn in one of these two chances, take out your capture net, and capture it. Once caught, the achievement will unlock.
If the bird does not spawn at all, you will want to fast travel back to Astera, and then start your expedition process over again to see if it spawns in. Rinse and Repeat until it's available to be captured. The bird is pretty rare, so it could take you a while before finding it, but the entire loop described above, shouldn't take you longer than 2 minutes and 30 seconds.
You can see the video for further information.