In Mission 5, wait until your target collects the intel. He will start walking around the map. Follow him until he arrives at a fountain. On one wall is scaffolding with a rock on top. Wait until your shot is covered, then aim for the explosive barrel at the bottom of the scaffolding. The construction will collapse and bury your target, and you will get the "I Siwa You Did There" achievement. If you get spotted and he stops walking (and will not continue after you hide), try attracting him to the location step-by-step by throwing stones.