(Only on Roche's path; Chapter 2)
This achievement is best done on Easy, but it's attainable on Dark (although you will have a much, much harder time). In the Kaedweni camp, you will notice a large arena in the middle of the area. To actually be able to participate, you need to complete the "Butcher of Cidaris" side quest.
To begin this quest, approach the arguing father-son pair in front of the canteen, and a small cutscene of their argument will ensue. Approach the father after the cutscene, Manfred, who is drinking vodka alone. Hear his story out and agree to help him. You will be tasked with assisting his son, Sven, in an arena fight against Letande Avet, the Butcher of Cidaris. Upon following through with the quest and killing Avet in the arena, speak to Manfred. He will offer you information to continue the main story, give you a square coin, and complete the quest.
Sven can actually die in the arena, and although Manfred will be very upset, he will still give you the information about the "Conspiracy Theory" main quest as well as the square coin. Needless to say, the arena will also still unlock.
After the quest is done, speak to Proximo (he will stand right by the arena during the daytime) about participating. Accept the rules and the fights will start. The fights are all pretty straightforward, but if you're playing on Dark, follow the strategies you've been using up to this point (Quen, Igni, roll around, etc.) and you should be able to clear the arena without much issue. The last opponent, Ves, is much harder than the other opponents, so keep that in mind. Upon defeating her, talk to Proximo for your reward, where your achievement will finally unlock.