This is the final stage, where you defeat the enemy that captured your family. In single player, the stage is best played with the shotgun and nitro.
When you head left, you will see a large set of blue faces. At most, they move up and down, but there are some that remain off-screen and can ambush you if you are unprepared. These screens are easily killed with the shotgun, with the splash effect destroying them quickly.
Occasionally, you will see a red face, which will shoot a bullet. They're easily avoided if you were clearing away enough space.
After crawling through the tunnel to the left, you will encounter a large head. It's main threat is to spit out crawling leaches as you, as well as a back and forth movement. The shotgun will stop the crawlers from approaching, but the three-way is better for taking care of the ones that slipped past the shotgun.
When it approaches, move to the back corner. If you're using the shotgun, aim just below the nose, to stop the green crawlers. As part of its pattern, it will widen its mouth and drop a large number of crawling brains that are easily suppressed by the shotgun.
When the head takes enough damage, the brain inside the head will continue the attack ans spit out the green crawlers. It's safest to destroy the crawlers first, then when you have space, attack the brain itself.
When the brain is killed, you are shown the ending, with the members of the family you rescued.