Warhammer 40,000: Inquisitor - Martyr

Champion enemies are tougher than regular enemies. They're still pretty common, and you'll encounter about 15 each mission. They will say "champion" under their health bar, and they'll usually have two health bars.

I unlocked the in-game perk for killing 250 champions the mission before I unlocked the achievement. So it looks like the achievement sometimes fails to count a champion kill. Just keep going and you'll get it soon.