On the act: "Requiem for a Gunstringer" you must complete it without losing a heart. Your health is measured by multiple hearts. However, every time you are damage it only does half a heart. However, even if your heart becomes broken in half it still counts as losing it so you can not be damaged at all. First clear out the three skull walls and progress to the bridge area, as you make your way across the bridges hold right, left, right, and finally hold left one more time to make it pass the bridges, don't forget to jump at the tip of the bridges. If you are having trouble with these just remember to move your hand slowly and smoothly, same goes for the rolling heads and flying coffins. Now for the two purple fire trails simply stay in the center and zig zag between them. You shouldn't have any trouble with the puppet skeletons that fly at you but, if you do simply be aiming at the coffins before they come out. This achievement is difficult and will mostly likely take a few tries.