The Uzi can be found in "Egypt - Sanctuary Of The Scion". When you reach the big room with the Sphinx, you have to climb up the ladder directly ahead of it, and have to choose to go left or right. Go right, and continue on until you get to the part where you are attacked by some weird flying fire creatures. Kill them, then grapple across to the top of the Sphinx's head. Once there, pick up the Uzi for the achievement.