The hearts are the black cirlces with a red heart in the center. You have to shoot them to collect them. In order to get cash instead of health, you must have full health when you shoot them. It is suggested that you try to complete this achievement in one of the earlier acts as they are the easiest. There are normally only 3 hearts in a single act. Just keep practicing until you get the level perfect. If you practice the method for boosting money you may be able to unlock it on that level. However, if you wish to hit two birds with one stone, this achievement can easily be obtained while going for the "Undead and Untouchable" because you can not be damaged once and there are four hearts on the level that are easy to see. The first one at the first purple fire. The second one on the long bridge on the right and the third one is right after the second except one the long bridge on the right of the second set of bridges. Finally the fourth one can be found midway through the second purple fire.