Aragami

This is easiest to obtain in versions of the game prior to v1.04, as prior to this, all kills countered as 'stealth kills'; post-patch, if a guard has detected you, you'll just be credited with a standard 'kill' if you kill them.

To obtain this trophy, you need to trigger a guard attacking you while also achieving a kill yourself at the same time. Before update v1.04 this could be done easily with a single guard, simply attacking them at the same time as they attack you. After this update, you'll typically need to involve a second guard, manipulating them into attacking you at the same time as you kill another, unaware guard.

Here's a video guide, courtesy of Trophy Germany: