I found that this works on the level that you have no epuipment(don't remember the name), find a guard that is unaware of your presence and pick up an object. Aim the object at their head and throw it at them, if they do not know that you are there they will be knocked out, I'm not sure if this works on aware guards, I am still testing that, I will post an update when I figure it out. Suggested objects are either cans or bottles, other objects may work too.
The best time to kill Hisham is when he in the parking area. If you shoot him there you can pick his body up and walk to the red car and dump him in the back. Then press 'Place Charge' and step back and detonate it and the 'Opportunity Objective Complete - Dispose of Hisham's Body' will appear on screen.