The Witcher 3: Wild Hunt

Eredin is a tricky little bugger if you're not careful. You'll encounter him in the final mission of the game, after you dethaw when Ciri breaks the Winter magic staff causing the great freeze to end.

My best advice is to use the "hit and run" approach, as that's what I used. He hits hard, and he is capable of dropping something like ice bombs on you, so lots of bobbing and weaving is necessary. I would run up to him, blast him with Igni, throw a couple of Strong Attacks at him and run like hell away before he could counter. He is also capable of moving from one end of the ice to the other in a blink, so constantly keep moving so you don't take a sword to the back when he pulls his little Houdini acts of his.

At about half health he is going to drag you through a portal where the majority of the rest of the fight will occur. Again, hit and run is your friend here.

When he is down to about 10% health he will disappear and you will have to follow him through the portal one last time. Another hit or two and you'll enter a cutscene of Geralt slicing him across his gut and Eredin will reveal to you the secret he promised at the beginning of the fight.

When he informs you that you have been betrayed, he will die and the achievement will pop.