The hardest mission. The authors of game definitely make this part too hard, so I give you the Cheat (write this in menu cheat and click enter):
Repletion the vital force of main hero - MEDIK
Thanks to this , you have chance to do this mission.
Kill three guards before you and take their guns. Kill enemies, who are behind you. Then kill the opponents, who are above you. Go through the gate.
Go to the Sheffield and kill him. Take his bazooka, go to the main gate and fire to the centre of gate. Go through the hole.
Chase Gaspar. Turn right, go upstairs. Be careful, there are many enemies... Go to the bedroom and kill Gaspar. In this moment rest of enemies can't regenerate anymore, so kill them.
For this moment you have been waiting very long... Go to the room, there is Sosa.
Use M16 and kill Sosa. Then watch outro... Congratulation!
THE WORLD IS YOURS! AGAIN...