This secret achievement is for defeating the Great Balor. This will be the first quest you get when you arrive at Mel Senshir. You will want to stock up on potions (as always), and just make sure you are prepared for fighting a few tough enemies.
The first part of the quest shouldn't give you too many problems as you're just fighting a few tuatha archers, warriors and priests. You'll want to keep a fateshift charged for later in the quest so avoid using it if you can.
When you finish watching the cutscene of the fight, quickly dispatch of the two archers and then concentrate on Malwyn. After wittling down his health a little bit he'll jump onto the back of the great balor, and at that moment (definitely if you're playing on hard) take a health potion, stand at the back of the platform and block with your shield directly against the direction of the Great Balor's yellow laser. Now just rinse and repeat until Malwyn is defeated.
Defeating The Great Balor:
This boss fight is very easy if you know what you're doing. The Great Balor has three different attacks: A laserbeam attack where you should run to the front of the platform, A sweeping attack where you should run to the back of the platform and a breath attack which you should block with your shield. After avoiding these attacks a couple of times he will slam his leg into the platform which you should begin to attack with a sword. After dealing damage to his leg, he will slam his head onto the platform and now you should use the fateshift you saved from earlier, and attack his face. His health will plummet and you should be able to kill him within the first time of doing this. If he has health left then rinse and repeat. Easiest boss fight in the game and you are rewarded with a 75G achievement.