I've found out the best way to get this stunt. When you're climbing Alden's tower, you'll eventually come face to face with a guy who shoots grenades from a launcher (the platform on which 3 guys come out of a container). That, you already knew. However, if you climb on the street lamp that is farthest from the elevator (near where you'll have to climb next), the grenade guy will go crouch inside the opened container. What you have to do is to launch a shockwave the MOMENT he shoots a grenade. The 'nade will fall at his feet and he'll die. Bingo. Stunt achieved.