Warhammer 40,000: Inquisitor - Martyr

A Killstreak bonus is achieved by killing 15 enemies within 3 seconds of each other, i.e. 3 seconds or less between each kill. Do it thirty times.

You don't need to work on this achievement. It's probably one of the first ones you'll unlock.

Continue your streak to 30 kills for the Massacre bonus. Do that one thirty times as well for another couple of achievements.