First the mongoose hides behind the central missile/rocket, stay on the
other side of the rocket (where you climbed down the ladder) and when he
appears blast him with your twin mini guns, also try and hit the missile
side where he is and some gas will come out when he breathes it in he
will go back to his side and start coughing (you will see it in the box)
when that happens go round to his side and blast him, if you run out of
ammo don't use the assault rifle use the shotgun. When he has taken
enough damage the central missile will go down and will say "think your
smart then find me". When this happens go around the perimeter of the
area until you find him. Now is the time you use the a. rifle spray him
in the face until he stops moving then launch 1 ar grenade AT HIS FACE!
If you do this he will die instantly whatever the difficulty! I know it
sounds complex but take my word its not!