i have heard a lot of complaints about that level and i know how to do it. first, when your on the rooftop, try to get full adrenaline and use it on the nemesis and his people. I suggest using the M9 bazooka on them. then, when they are all dead, go up to the room and go into the second room there, then go up to the microphone-like thing and press x and it will say "bomb stopped: mission complete" or something like that. and then, your done!