Dead Nation

At the end of this mission, you'll reach a graveyard with 4 mausoleums, if you are like me and don't really understand the mouth monsters, you will die here, In each of the four stone buildings, there's a mouth monster, and they will call an infinite number of enemies. You need to blow up (or shoot through) the stone walls, and kill the mouths. As you are doing this other enemies (and "boss" monsters like cutters) will appear.

Eventually you'll have to kill all 4 mouths that are "hidden" (But visible to the player) Otherwise the end of this mission will have a never ending stream of enemies.