This achievement is Getting a Head worthy of 15G
This is story related and is unmissable.
As you can probably tell from the achievement icon, this is for cutting a Hydra's head off.
**Spoiler**
To start this quest off, talk to the Captain in the area and she will tell you, you should rest to gain your strength. When you do this, you awaken to a Hyrda attacking the encampment. You don't have to fully defeat the Hydra, just chop off his head. It is virtually impossible to die at this part, since the Hydra will be attacking the rest of the encampment. When you defeat the Hydra, his head will drop to the ground and you will get the achievement after you talk to the captain again.