This boss is toward the end of Chapter II, during "The Eternal Battle" main quest. At the end of the quest, you are faced with killing the general of the =enemy army. You do control Geralt here (contrary to the other spots in this quest where you control a ghoul), so you can cast your signs and roll around, etc. For me, this boss was actually much easier on Dark than on easy.
The way to approach this boss is to keep relatively close to him and bait him into coming at you with Yrden. He will charge, but will get stuck on Yrden, allowing you to get some attacks in. Rinse and repeat and you;ll take him down. On easy, he will resist Yrden twice, but will get stuck a third time, allowing you to once again do some damage. However, on Dark, he doesn't seem to resist Yrden at all, getting stuck on every one that you plant.
If you back up too much, he will hail his archers to rain arrows down on you. These do a lot of damage on dark, so keep rolling for the duration of the attack, and then roll back in close to plant an Yrden to bait him into.
Upon the general's defeat, the achievement will unlock.