Mosh Pit Achievement in Assassin's Creed: Revelations
Have 10 guards poisoned at the same time.
How to unlock Mosh Pit
You will need to create a bomb that spreads poison for this achievement. To make the bomb, you must have ingredients to create it. Any shell works technically, but impact is best. Use the British Gunpowder to ensure widest area of effect. The main ingredient to even make the bomb is Datura Powder. You can get these ingredients in many ways, looting dead bodies, chests, rewards, etc. You can even just go buy them if you can't manage to make them.
Focus on crafting both a Cherry Bomb and a Datura Bomb (the poison bomb's name). Find an area with a lot of guards and throw a Cherry Bomb in the center. You may also use other forms of distraction based bombs as well. Of course, there are other ways but the ones listed will make it easier.
An excellent spot to get this is in Sequence 4 Memory 1. You'll come to a courtyard after knocking out 6/9 people and there will be several guards. Make sure you have a Cherry Bomb. Climb up a little bit or stay out of view. Throw a Cherry Bomb to get the guards to one spot. Now throw your Poison Bomb in the center of them to gain the achievement.
Here's a video for further assistance: