This is the simplest way to accomplish this achievement, without much effort at all.
This achievement is best obtained in Sequence 3 Mission "Vanishing Slaves".
After you race Elise to the slavers you will climb a set of trees to a platform. Once there Poison dart the third enemy to the left and then Berserk dart the enemy in white. Now all you do is wait on the platform and just wait for the fight to finish.