This area is similar to the Black Rider stealth sequence. If Boromir catches you, keep pressing Circle to resist. Walk to the right, pick up the item, move up, throw the item at the branches. Continue on the path. Crawl under the small hatch near the stairs, then push down the pillar. Drop down and build the stairs. Walk up the left flight stairs. Smash the rocks on the left side of Boromir, pick up the item, then throw it at the gray columns above. Build the stairs. Walk up and hit the birds, then drop down. Continue to the right. Smash all the objects near Boromir, including the pillar to the left, and build the catapult. Hit the catapult to make it fire a boulder. Follow the path upwards. Pick up the item along the road then throw it above Boromir. Continue along. Hit the small rock near Boromir, pick up the item then throw it at the upper left area. Build another set of stairs. Walk up and push another pillar. Go down and follow along. Enter the small hatch left of Boromir. Hit the rocks on the right, pick up the item then throw it at the arch above where Boromir is. Follow the path upwards then resist the Ring.
Head to the rightside area, smash the wooden oar, pick up the torch, light it up in the fire, then use the torch to burn the plans and branches blocking the path. Switch to legolas, jump up the elven platfor, jump up the tightrope, hit the target. Build the rubber steps. Use Gimli to smash the rock blocking the way. Continue upwards. Throw Gimli at the pillar to the right. Use Legolas to jump to the pillars then shoot the bars. Jump across the bars. On the high platform, push the pillar to the bottom. Use Gimli to jump-smash the gray floor. Push the two pillars near the upper ledge so they will act as steps next to each other. Jump across and continue the path. Smash the huge log using Gimli, throw Gimli at the rock above, then assemble the pieces. Switch to Legolas, jump up the fallen log, shoot the bar, jump across the bar to the upper area. Assemble parts again, then push the pillar down to the lower area. Use Aragorn to pick up the green-glowing sword. Follow the green mist until Aragorn crawls and finishes his tracking by the bushes above.
Beat down the Uruk-Hai on the ground using your melee, then for the ones atop the pillars just smash the pillars they are standing on first. Continue beating up the orcs until you reah the last area where you need to throw Gimli at the rock first. Then climb up and beat the orcs on top and shoot the orcs on the top left ledge.
If your hobbit gets caught, switch to Boromir then attack the orc holding the hobbit. Simply go to the upper leftmost area and crawl under the small hatch using a Hobbit. Attack Lurtz. The second part is the same except you have to clear out some plants blocking the small hatch first. For the last part, go to the rightmost part, build the plank then jump up and attack Lurtz.
Wait for Lurtz to throw his shield, jump down and get stuck. Attack him like crazy. If he catches you, switch to the other character and hack away.
Follow the stud trails to trigger the next cutscene.