After acquiring the Yellow Sign, play through Cappella Magna until you reach the 30 minute mark. At that point, the final boss for the stage will appear. The boss will telegraph its moves by placing down red strips vertically or horizontally before engaging with each attack. Watch for the red zones and make sure you have room to move around. Always move away from the boss and the next zone. After defeating the boss, the achievement will unlock, you can collect the Great Gospel relic, and the Game Killer arcana will be available.