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Now here in this part of the second level, you're in a wide-open area that is crawling with enemies. First, shoot all the bad guys with the turret gun, more will come in after this - most notably a particular one that pops out of a doorway on the platform across the tiny gap, he likes to throw grenades. You need to take care of this guy in advance otherwise you will never have a chance to shoot the lock on the gate.
So here's what you need to do... Simply shoot at the one who pops out of the doorway across the small gap 'while' you are fighting the other enemies as opposed to focusing entirely on him. A new one will take his place approximately three to four times (don't bother counting, you're going to be busy shooting all of the other ones anyway).
It would probably be best if you eliminate the gunners beforehand since they will definitely do the most damage as opposed to the ones performing weaponless combat. In general - 1) Use your low level ammunition first before using your heavy ammunition; 2) Make sure you are in constant motion and not just standing still like a sitting duck; 3) Always remember to make use of the lock-on button (L1) with the trigger button as opposed to just firing into the darkness, eventually running out of ammo and dying.
