This part is hard. There are so many Locust here it is impossible to tell how many there are. It is just one big fire fight. Start by heading straight to a wall to take cover behind.
Just shoot any Locust you can. Emergence holes will open. Shoot the Locust that come out of them. If you can seal the holes with frag grenades do so. At some point an emergence hole will open in the building across the way. A Boomer will come out of the building. Kill him quickly. Mop up any remaining Locust.
When it seems that you've gotten all the Locust and the fighting stops, you haven't. An emergence hole will open behind your squad. It will have 4 boomers. Kill them very quickly.
They can kill your squad in nothing flat.
After the fight a cut scene will begin. This will bring Marcus to the end of act 2.