Head for the Stormcloak camp near Solitude. Talk to Galmar and he will send you to Fort Hraggstad. This operates exactly the same as Snowhawk did. To finish this quest, you must clear out all of the enemies. You will be regularly updated with how many enemies remain. When it hits zero, no more enemies will respawn and the quest will be completed but there may still be enemies remaining. They will likely run away at this point though. This time, report to Galmar when you've cleared the fort, rather than Ulfric.