Here's another fun fight. Go join the troops south of Fort Snowhawk and you'll begin an assault on the place. You have about 9-10 troops with you, and your goal is to kill all of the Imperials at Fort Snowhawk.
Instead of killing everyone there though, you have to kill everyone who's there AND everyone that the game spawns in as reinforcements. To keep track of your progress, the game represents the Imperial force as "100%" and for every few Imperials you or your fellow soldiers kill, that number drops.
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!
After the fight you'll be back on the "Liberation of Skyrim" mission, so go talk to Ulfric to continue.
Capture Fort Sungard or Fort Greenwall