Turgon believes defeating high ranking captains in Pelennor will give Minas Tirith additional time to prepare for the inevitable battle to come.
'Captains of the enemy roam between enemy camps in Pelennor Fields. I need to call upon your expert skills to defeat these leaders.
'Their defeat will cause much disorder within the different factions gathered upon the Pelennor.'
The high ranking captains of the Morgul-host can be found in Pelennor Fields.
You should defeat these high ranking captains.
Turgon can be found at the South-gate of the Rammas.
You should speak with Turgon.