To open, "Death Strike" is the assassination that can be executed while you're in sneak mode (L2), and approaching an unaware enemy from behind, pressing X when prompt to execute it. (By unaware, they usually have to be standing still with their arm's crossed, prime examples include Groms you encounter on Alsorna, and Varns found throughout Erimos (The Savannah)
You will need a dagger to execute the attack, they can be easily found or looted from enemies.
This trophy can be tricky since there is a limited amount of enemies that are properly set up for the attack, so it's recommended that you carry a dagger with you at all times, just to be ready for when the opportunity arises. (Also to note, if you spook some enemies into attacking you, you can run away until they're out of sight, and they should return to their original position)
You'll discover many Varn camps throughout the savannah which are prime spots to rack up kills for this, most contain a guard or two that is standing around the surrounding area, as well as a few groms while wandering around Alsorna.