This battle is a LOT like the previous mission ("The Battle for Fort Snowhawk"). In fact, it's damn near the same. You'll join 6-7 of your fellow Stormcloak soldiers and will storm Fort Dunstad. I'll give the same advice here as I did there:
With that said, its your job to go on a killing rampage. You'll have to storm the front of the fort first, where barricades will be in place with quite a few archers on the ramparts, but after that it's an all-out killing orgy. Kill as much as you can as fast as you can. Once you've brought down that 100% to 0%, the mission will be complete. There may STILL be stragglers though, and you can hunt them down as well.
It's a fairly easy (but fun!) mission since you have so many allies and can easily pick off Imperials when they aren't focused on you. Or, alternately you may wish to be in the thick of the action, as you can save the lives of some fellow soldiers if you are.
Note: I only had one ally die. See if you can beat me and have no allies die! Good luck! Yes, I had only one ally die on BOTH Fort missions.
After the fight you'll be back on the "Liberation of Skyrim" mission, so go talk to Ulfric to continue.