Gears of War Act 1 - Fish in a Barrel
After the cut scene some Locust shows up with a troika gun emplacement.
Flank the gun emplacement to the right. When Marcus comes around he'll be behind and left of the gun emplacement. Kill the Locust manning it.
Run up to the gun emplacement and man it. Turn around and shoot the 3 Locust that charge you.
After they're dead move forward, but expect another wave of Locust. Kill them as you make your way down the corridor.
At the end of the corridor is a big open area with a fountain. There will be a longish battle here. An emergence hole will open. Blast any Locust that come out.
After that one is closed, another will open. Kill these Locust as well. Once again, another emergence hole will open. Kill the Locust as they emerge. When done with that batch of Locust, they will make another emergence hole. Kill the Locust again. That should be the last emergence hole. You will find an area to the back and right of where you entered. Go ahead and exit through that area.