You will come across a Mountain Guardian (Nadaig Meneimen) during the main quest "The Sky's Teeth", but you if you want to get this done earlier on you do battle a Mountain Guardian as the final challenge in the Coin Arena in New Sérène.
Save your game before you start the encounter.
This Guardian has a ton of health, decent armor, a large sword and wings that allow it to fly around and swoop in quickly to attack you. I suggest taking out the Guardians armor as quick as possible so your companions deal more damage.
The Guardian can summon rock spikes from the ground in a straight line that deal heavy damage. Make sure to dodge out of the way as soon as you see these coming for you. If the Guardian hovers in the air for a moment, back up as it’s about to spin in a circle damaging everything within melee range. If you’re a mage then the Guardian should mainly focus on your companions while you keep a distance and DPS. Just be careful as the Guardian can fly up and come at you at any moment. Remember, if you’re having trouble you can always change your difficulty to Easy.
Once you kill the Guardian you’ll obtain the achievement for defeating the Mountain Guardian as well as the achievement for Completing all 5 Challenges in the Arena.
The Arena Master will also reward you with some good and a Unique two handed sword called “Broadsword of the Deceased Kings”.