Unfortunately, I'm not 100% certain of this, but this seems to unlock as long as you kill an enemy with a yellow health bar. The first one of these to appear, barring a random event, will probably be the Archon during the "Restoration" mission (mission 2). At the end of the mission, you'll face off against an Archon.
Technically, it may be possible to not get the kill on the Archon and not get the trophy. You can always replay the mission, but you're likely to kill quite a few of these Champions while you play so you can just wait for it to happen naturally.