For this achievement, load up the Piri Reis Mission: Caltrops and throw the caltrop bomb as instructed. Then, run up, duel execute 4 guards, and quickly execute the last guard.
For me, it helped that I aimed at the center of guards instead of aiming at an individual guard. Move from right or left and do low profile double assassinations for first 4 guards and then a high profile assassination for the last guy. For the last guy, you may have to make sure that you're hammering that high profile assassination for the fifth guy while you're still dual assassinating the last pair of guards, or you'll never do it in time.