... I lost the main guy several times as well.
The mission description says Ogers lie in Ambush ... but you simply get invisible enemies.
(Small squads which you cannot see or target while they live. When dead, look humanoid. Strong in melee, weak against arrows, apparently.)
I brought three or four squads of Goblin archers and three Marauders squads (plus some melee).
At the beginning of the mission, the quest guy runs off towards town, into the first ambush. Send your guys right after him.
You won't be able to click on the enemies, so to keep the quest guy covered keep moving your archers and telling them to Stop (Hand icon) near him. When they fire at the ground and corpses start appearing, you've found an enemy.
Once the first ambush is done, you get a short pause (several minutes). I kept my melee and Marauder units next to the quest guy, and used the 4 Goblin archer squads to clear out some more invisible enemies. Be wary of ambushes, the invisible guys seem to move around the map.
Enemies, from what I can tell, move around the map while cloaked and try to reach the quest guy. After a while, possibly after you've hunted down all the stealthed mobs, he moves to the next location. Keeep the quest guy covered until he reaches town (or possibly until you've found all the enemies).