Ok, the 5th mission, Three Kings is set on three hotel rooftops.
The first hotel has grenades. I found that throwing my own grenades back a him worked great. Just keep zipping around the roof with your LB device.
The second hotel is killing the russian in a tank. I found if you go behind the platform, no C4 goes off, and he can't hit you. Use your Zipline to hang on the back wall, stay near the corners. DON'T JUMP UP. just hang with your head bove the wall. When Sheldon (helicopter pilot) fires at the tank, it turns away from you, letting you plink headshots. Feel free to drop down when he turns back toward you. I killed him with a pistol no problem after several joint attacks.
The third was just as easy, run behind the stage, the missiles will hit him. he may go flying and not get hit a round or two, but just stay in the back and let the missiles do all the work. Remember to keep an eye out for adds that will run toward you on this stage.
This whole mission took several tries, but once you find a crack in the armor it goes smoothly, until the next check point :P
I hope this helps