This is only obtainable on Roche's path of the game, so keep that in mind.
Towards the end of Chapter 3, before the last quest, you are presented with another choice: to rescue Triss Merigold, or to meet up with Roche to kill Dethmold and rescue Anais La Valette. I strongly advise doing this anyway, especially if you think you may not hit level 35 by the time you do the "A Summit of Mages" main quest (if you convince Roche to take Anais to Temeria rather than giving her to the Order, they will attack you and then you can farm them in their camp outside Loc Muinne).
The mission is fairly straightforward but there is a lot of combat. You will be fighting a lot of fully armored knights, and you will also want to use Quen and Igni to keep them at bay while also doing a fair amount of damage. When you get to Dethmold's quarters, Roche will kill him and you will proceed to take Anais. After you return to the starting point of this quest (whether you fight or are escorted by the Knights of the Order), the achievement will unlock.