Note: This mission is much easier with the Shadow Strike skill. Things begin with yet another journey to Shelob.
Your new quest requires you stop the fall of Minas Ithil via a human traitor. To track this quisling down, Gollum helpfully leads you to his orcish liason. Follow the little mutant through the caverns, and he'll soon lead you to spot where you can learn to shoot spider nests.
Shooting the nest summons orc-hating spiders. This batch will attack a convenient incoming patrol, though you're welcome to hasten their deaths with you own blade. Once that's done, sneak after Gollum to the fortress gate, just in time to seek the messenger you seek slip inside.
There's bonus for not being detected, so veer, left away from the main gate, and spring up that low section of wall between the guard towers.
Kill the archer on overwatch here- shadowstrike works well, though you can also use a simple headshot, and jump across via the lower roofs if you need to.
There's a bit of elfshot right next to that newly-dead archer, and the elfshot wall is the perfect place to eavesdrop on the messenger (he won't move on to the next objective until you eavesdrop, and you have be close, with a clear line of sight to trigger the eavesdroppery).
The ropes by the lower roofs here are tempting, but it's too easy to get spotted. Instead, keep to the heights. You can Shadowstrike the archer overlooking the bridge, or just stealth your way along the ridgepoles.
Eavesdrop on the messenger again, who's at the near side of the bridge. When that's done, use the ropes to cross the cavern (there are various nests and traps to deal with the bridge guards, but they're unpredictable, and increase the overall chaos + alertness).
Scale the wall right next to the gate for a vantage on your final conversation.
Whether or not you got spotted en route, you'll still have to fight this warchief to complete the mission. If you've been stealthy, you can thin out some of his archers and spearman before moving down to confront. Softening up a caragor can also be useful, if you have the skill to ride them.
Likewise, now's a good time to summon Gondorian backup, if you have it available. If you find yourself hard pressed, the tall walls and towers in this area make dandy quick-escapes for you to catch your breath. Whichever methods you use, the battle will end with you interrogating the warchief for further intel.
This doesn't go quite as expected, but you still complete the mission, and draw your rewards.