You gain the poison ability in DNA Sequence 5 during the memory ‘A Blade with Bite’. Once this mission is complete, select the poison by holding and moving the to the top left of the weapon wheel to arm yourself with it. Find a person or a guard, it doesn’t matter which, standing near a climbable box or wall. Walk up to them and press to release the poison. They will begin to stagger around. Climb up the surface and when the icon appears to assassinate them, press to jump down on them to kill them.
As soon as I got access to my poison blade I went out in search for this...a few mintues later I found 3 guards wandering open ground and there was a pillar there. I poisoned one of the guards, ran up the pillar, jumped off it and killed him! First time I poisoned someone and I got it...lovely :)
I done it at Venecia carnival. After tournament for mask, go to next memory - bordelli. At the entrance there will be pack of guards. I poisoned biggest one, get on box next to him, and jumped. Run as hell while enjoying achievement.