There are 3 parts that you have to complete in the level HELM'S DEEP: THE DEEPING WALL. In all 3 parts there is a GPS on the top-right corner of the screen, which shows the walls of Helm's Deep. Slowly, red dots will appear on the GPS, which indicate ladders that are being used by the Uruk-Hai. Make your player go to the ladders according to the GPS and kick them off with the CIRCLE button. In each part you have to do this, but it takes longer and becomes harder. So don't bother about killing Uruk-Hai.
1st part: takes 2 minutes (few orcs)
2nd part: takes 4 minutes (some orcs and fire tipped arrows)
3rd part: takes 5 minutes (many orcs, fire tipped arrows, and explosives)
Gimli is the best player to use for this level. He easily kills the Uruk-Hai with the TRIANGLE button. This gives you more time to kick the ladders off.