A: Roemer is actually very very easy. All you need is one gas grenade to kill him. Keep throwing them until he gets caught in the gas and dies.

A: After grabbing the C4  climb back up the ledge and go left to the guy who was throwing grandes at you. You should now see a red box, first kill the guy above you, then climb the box make your way all the way up. After climbing up, go straight until you see a door with bars, you guessed it put the C4 on the door and RUN! From there you should know what to do.

A: Yes. Visit Syphon Filter codes. 

A:When you follow the scientist, there will come a time when you will have to kill two guards. After the scientist finishes talking to them, throw a Gas Grenade and after that you have to walk while you are crouching down. While you are walking make sure the scientist doesn't run, he has to walk. If he runs, don't bother playing because he won't open the door, so just restart the mission.After that there will be a guard around the corner kill him when he starts walking to the place where the ramp is, shoot right away when you see him, then just walk up to the scientist and you've got yourself a checkpoint. 
I give a credit to Mike Bruder for finally sending me the answer to this question.

A: Use the sniper to shoot her gun.

A:Oh finnaly I got that one, I had at least 50 emails for that one. Alright, remember the elevator right next to the rocket? When you are on the middle floor shoot the broken elevator switch on the top while you are standing on the elevator and it will go up. Over there you will kill some guys and you'll find the card key from there.

A: It's located in the valley where the elevator is(in the alley. You have to jump on the box near the fence, get close to the edge and press triangle, Gabe should now jump on to a fire escape, now look up there should be a pipe that Gabe could jump on, as you make your way to the other fire escape you will see the grenade launcher.

A:The explosives also known as C4's can be gotten after Lian contacts you while you're on the train. After that fall down and kill the two guys that are running on the other side. After you kill them there should be a guy throwing grenades at you or shooting you with his shotgun. First kill him by getting near him so he would drop the grenade, then run. And the grenade should blow him up leaving you alive. Now go back where you jumped down from the train (crossing the side again if you haven't done so already) then turn on the flashlight and walk down the railway and you should see a white box, open it and you got C4.

A:Although Lian and the other agents can't find his weak points, Girdeux is actually very easy to kill. Girdeux's weak point is shooting him from the back hitting his tank. Hitting his fire tank is the only way to kill him, and you should only use a shotgun(it's most effective), Always stay behind him and never let him see you, so hide behind the poles all the time. Shooting him 3-4 times should set him on fire and give you the victory.

A:The first two are very close two each other so I think you should have no problem on them, the first one is located in the second square bush(look on the map if you see a blinking object go to it) the second bomb is behind the statue, the third is northwest of the statue, in an area with alot of terrorists. The bomb should be located near a little building. Now head back to the statue and go north(be careful there are guys hiding on the top be ready) after that follow the pavement until you see the last bomb, protect the CBDC agent and voila the time is gone and you have just found the fourth bomb.