To unlock this achievement, you must defeat a Manticore.
Manticores are rare creatures which can be difficult to find in Aurai. However, there are a few places where they can be reliably found. These are Parched Shipwrecks in Abrassar, and south west of the Cabal of Wind Temple in Enmerkar Forest, as shown below.
If the Royal Manticore proves to be too much trouble for what it’s worth at this point, the most reliable place to find a Manticore is in the region of Abrassar. They roam throughout the region, but there is one place in particular where you are certain to find one. Better yet, it will be weakened after fighting two other creatures. The location in question is known as ‘Parched Shipwrecks’, and is north west of the city of Levant.
When you find the Manticore at the Parched Shipwrecks, wait for it to finish off the two assassin bugs before you initiate the fight. By the end of the confrontation, the Manticore should have lost at least a quarter of its health, making it easier for you to take down. As with the Royal Manticore, be careful to avoid any elemental bolts that it fires. Keep your distance whenever possible and make use of ranged skills. You should be able to whittle down the health of the Manticore without much issue, so long as you preserve stamina use efficiently.