Launch this mission through the marker at the tower gates.
A cutscene plays, and when the action returns you're tagging along after Idril again, to defend the upper city gate (again).
Do watch out for the bombardment icons, and warning orange rings- the explosion shere really pack a punch. Elven sprinting and shoulder rolls can get you past the ones you're unable to avoid.
There's a minor skirmish at the gate- cut down these nameless no-hopes and follow Idril again, this time to a siege beast. She and her friend will suddenly remember they have someplace else to be, as you face the thing alone.
Luckily, this first one's a real pushover- take aim at the light blue areas of the beast's armature and it'll go down with just a few charged shots.
Hop over and Dominate the poor thing (where are its handlers?), then turn the tables on the invaders.
Take out the swarming orcs below. Don't worry about your own safety, they won't climb the tower after you, but do remember to lead your shots- it takes several seconds between launch and impact. This isn't an issue for taking out the second siege-beast- just lob fire rocks at it until it's dead. When all opposition is crushed you'll have destroy your own siegebeast before moving out. That's a little grim, but just hold down L-CTRL and Talion will do all the dirty work for you.
Now you need to get to the next siege beast before you timer expires. This is made easier by the sudden addition of the Shadow Strider skill, which gives you a generous douple-jump capacity.
Use this in combination with your Elven sprinting to cross the cityscape. The time limit isn't as tight as it seems- you just need to be in range of the next Beast, not actually atop it when the timer runs out.
Once you're in range, everything plays out as before. Shoot the weak spots, dominate the Beast, bombard the enemy, and then disable your unit.
This ends the action, throwing you back to another cutscene in the tower.
Completing the cutscene completes the mission. This also adds the new collectible type 'Lost Artifact' to the map (there are 11 in Minas Ithil), and opens up the Cirith Ungol territory for further questing.