When Mission three: The Good Doctor begins, go straight to the rooftop of the hospital. You can either use the elevator or your traversal powers to get up there. Stand on the arch (edge of rooftop) and jump down onto an enemy. You must kill the enemy by pressing [Melee] just before hitting the ground to get the "Fearless Fall" achievement. You can lure enemies to specific locations by shooting the crossbow or throwing an explosive (the crossbow is best for this). The enemies will investigate the impact point of your crossbow bolt. This makes it much easier to get them in a good position for you to jump on.