When you get to the pit you should take the Desert Eagle handgun, now you have to exchange your rifle with the M9 hand which is on one of the ammo boxes in front of you.

Now you have 2 handguns, run through the pit with your m9 equipped.

You can sprint faster and move faster while aiming than when you have the rifle.

Now run and shoot the first 3 targets then sprint forward and shoot the next 2 targets, then sprint again and keep moving while aiming and shooting but be accurate.

Keep this way and remember to reload your pistol before going upstairs.

Keep shooting and moving until the end and remember switching to your side arm is faster than reloading.

It's very easy, the trick is in the sidearms!