Aragami

In many of the game's chapters, you'll encounter floating spheres of light. Approach too close to these and they'll attempt to attach themselves to Aragami, then detonate within a few seconds. However, if you can find and attempt to attack a guard in their period before the explosion, you'll use this explosion against the guard and instantly kill them - earning this trophy as you do so.

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