You'll begin this mission as you do so many- catching up with, then tagging along after Eltariel.
When you do catch up, your goal is to kill the 9 orcs in camp here. Which isn't very hard, but is a nice chance for the two of you to work together.
Now you wait for the procession, ready to kill the orcs as they enter camp. There's a side-goal to kill more orcs than Eltariel.
The best way to do this not wait for the orcs to come in sight of her perch on the tower, but bring the battle to them- running up the pass to execute them. If you see any stray into Eltariel's range, use your bow for a bit of friendly killstealing. Otherwise, go wild.
Interrogate the messenger toward the end of the procession for word on the captains. Once you've completed the interrogation and killed any lingering guards, your side-bet with Eltariel ends. Rejoin the elfmaid in question, then go to face the three captains.
This can be a trickier fight than it seems because there are no grunts handy to dominate, or drain, or steal counters from, or use as shields, etc. Focus on battering the enemy down one by one, but prioritze any captain using ranged attacks- they always strike at the worst possible time.
For all her bloodthirst, Eltariel doesn't actually care whether you kill or recruit these guys, so do as you see fit until the last one falls, launching the cutscene that completes the mission.