There are 3 bosses in the game that you can get this achievement on.
1. The Chieftain Strikes (The Desert of Kuneer)
2. The Queen’s Game (The Glauxian Brothers Retreat)
3. The Beaks (Allomere’s Fury)
The easiest boss is Allmoere, which is the last boss in the game. You should have the best battleset armor in the game, as far as health and defense go. Chase him after the initial portion of the mission, throwing hot coals at him to break down his armor. When you’re fighting him, just keep attacking him. Whenever he throws you backwards, be prepared to hit either or when prompted. Follow up with attacks of your own. You will most likely not get hit much at all in this battle. You will have to beat every boss without dying in order to get all gold medals in missions, but this is the easiest fight to get the achievement.