Foreword: Assault the Gates  

'Name, there are a pair of Orc-fortifications defending the banks of the Anduin: one which they call Durburz-stazg lies immediately to our north; the other -- Fallug -- a bit further to the south. One of our first objectives here is to eliminate these two fortifications so that our landing can continue unimpeded.

'I need you to bolster our forces attacking the encampments, particularly the assaults upon their gates. You must choose one of them to assault -- you cannot be in two places at once -- takes its gate, and hold it for as long as you can. We will send reinforcements there as soon as possible.'

There are two Orc-fortifications along the Anduin which are a cause of concern for Fuirgam.

Objective 1
Take the gate of one of the Orc-fortifications
Durburz-stazg lies to the north of Echad Sirion. Fallug lies to the south of Echad Sirion.

Fuirgam has ordered you to take one of the Orc-fortifications along the banks of the Anduin river.

Objective 2
Hold the gate against the Enemy
Do not surrender the gate

Objective 3
Talk to Fuirgam at Echad Sirion
Fuirgam is at Echad Sirion, on the eastern bank of the Anduin.

You should return to Fuirgam with tidings of your victory.

Go to Entrance of either Durburz-stazg or Fallug
When you arrive, you will receive a message, stay in that area, do not follow mobs into the Orc-encampment, just stay put for a few minutes, near the gate entrance, and you will get a completed message, then you can move in to do the other quest.
While you are standing here, you will be attacked by the Orcs, but you also have Malledhrim Soldiers there to help you, so just fight off the Orcs.