Instance: A Guardian's Duty  

Severin Wheeler has enlisted your help in the defence of Trestlebridge. The town is hard-pressed by the Orcs, and guardians are needed to keep them from crossing into Bree-land.

Speak to Severin Wheeler

Speak to Burle Pierson & Pierson's Son

Now speak to Hilton Harper

Talk to defenders of Trestlebridge

Prevent any orcs from reaching the bridge

Now you will see the fight start.

NOTE
Do not help the npcs here, allow them to fight the Tarkrip orcs, you are not after them, you are to defend the bridge.
With that said, you only have to kill one npc to complete this quest, to the northwest, one solo Tarkrip Raider will run to the bridge, if he makes it, you will fail this quest, no other Tarkrip orcs will attempt to go to the bridge, kill this orc, the Tarkrip Raider first.

Once the Tarkrip Raider is dead, go help the npc defenders kill the other 6 Tarkrip orcs, when they are dead, you complete the quest.

Now speak to Hilton Harper to leave the instance.