The best weapon at first is the rifle as it is endless. You are killing humans that don't take as many hits as the dinos. Watch out for the black guards. These are veterans and are extra hard. After the first gap, jump over some soldiers on platforms above you with flame-throwers. Shoot these from below before climbing onto that platform. After these you should carry on, up and right. Then change weapons to your Uzi as the enemies come thick and fast. When blowing up the fans, use dinosaur weapons. This will conserve your conventional ammo. To kill the main solider at the end of this mission it will take about six hits with the shotgun.