You'll get this naturally and fairly early in the campaign - for context I unlocked this at level ten, "Find the Chaos camp" main objective. Pick up the glowing red "mist" that drops occasionally from enemies - this will fill up a red circle in the bottom left corner of the screen, above the red health circle. Once it is full press LT to use bloodlust and unlock the achievement. The game will likely prompt you to use it anyway since it would be your first time using it.