For this achievement, I recommend choosing the Veteran with the "Voice of Command" skill or the skill that allows you to become invisible.
Unfortunately, you'll need to be less active in your group (let your teammates know you're working on this achievement so they understand you don't want to get too involved in combat).
When you see a horde rushing towards you, use Voice of Command or the invisibility skill (I personally succeeded with the invisibility skill because it cancels aggro, allowing the horde to focus on your teammates).
Prioritize assassination missions; they're short and have very few tense situations.