On the mission Sentinel you can stealthily kill enemies with your Grappling Hook while hiding in bushes. These kills can only be done in the stealth part in the beginning of the mission. When sitting in a bush attract them to you then kill them with your Grappling Hook. Be sure no other enemy or drone see how you kill someone. If you are seen the mission will fail and you have to restart from the previous checkpoint. Be sure you kill 20 enemies to achieve this trophy/achievement, there will be more than 20 enemies so its fine if you don't grapple all of them but make sure you grapple 20 of them.