Alexzander

Follow him until he gets in the chopper and then follow him and shoot him on the Happiness Island.

Moynihan

Dont take the mission for the money as you can only spend it on clothes. Then after he has killed Kate, you go to an old casino in Alderney. You pull up about 50 metres away from the building where there are about 20-30 guards which you must kill. After you have killed them you have to go inside the old casino and kill the 5-10 more guards inside then you go up a set of stairs to a room where Dimitri is. He then gets up onto the roof where there are a few more guards to kill. He then jumps down from the roof and gets in a power boat. You have to get on a Sanchez and chase him for about 1/2 a mile around the coast and then go over a jump where you grab onto a helicopter with Roman and Little Jacob in. Then you keep following Dimitri while you are flying the helicopter. He then pulls up at a dock and you have to land the helicopter and chase him for a little bit. Then he falls over in a treed area where you see a cutscene. In this cutscene you and Dimitri speak to each other for a bit and then you lodge a hunk of lead in his face.

Moynihan =)