This scene is right after the end of the Two Towers as far as movie-plot goes. You're chasing the Uruk-hai in the forest as they flee Helm's Deep.

Shoot a few of them, and shoot the carts to have them explode. Get the archers on the bridge from afar. Chase, chase, chase, and finally get to a ledge with a few to take out. Checkpoint.

You see the friendly neighborhood ents grabbing enemies. Go down the ramp to the bottom. There are Lots of enemies here, and a counter that says 75. Don't worry - the ents are helping so it's not THAT bad. You actually get to 75 pretty quickly.

Now a way is made to go forward. Go forward and kill more Uruk-hai. You work your way out to an area with towers - checkpoint

Work your way around to the left and up. Shoot another explosive cart and go forward. "Defend the towers" call the orcs. Kill the enemies. Use your magic when you can and take down the towers. You'll get to a tooth-like bridge across the chasm. Fight your way across.

Go across and left. Clear the enemies out then go back to the right and down. The camera angle is pretty awful here, top down through the trees. You get to a wheel - keep going down and around. You see the ents trying to pull down the dam. Shoot away his attackers. Then shoot away the support. Water will start to spray out after a bunch of hits. More attackers will come, fight them. Don't worry about running out of missiles - the enemies that come at you nicely bring you more. Soon you have 2 streams of water coming from the wall, and you can see the green progress bar lowering. It takes a LONG time but if you keep the enemies cleared, the ent will do most of the work for you. Help him when you are able to.

FINALLY you get the dam blown and see the movie sequence. End of level, and you get the Christopher Lee interview.

Return of the King Walkthrough

Lord of the Rings: Fellowship Review
Lord of the Rings: The Two Towers Review