This quest is likely as straightforward as the Shattered Sun Offensive dailies could possibly get. What is it? A kill quest. You get it from Magister Ilistar within the Sun's Reach Armory, which has become friendly at the conclusion of phase 2.
Magister Ilistar wants you to thin the ranks of the Dawnblade on the island itself. Specifically, he wants you to kill six Dawnblade Summoners, six Dawnblade Blood Knights, and three Dawnblade Marksmen. Their names are good indicators of what they can do. Summoners are warlocks, Blood Knights are... Blood Knights, and Marksmen are hunters. In my experience, these mobs are a little more difficult than the Wretched and Robots of the earlier dailies, but alone they still shouldn't be too hard. The only threat they truly pose is the fact that they are fairly tightly packed, so single pulls might be hard if you're the only one doing this quest. However, Quel'Danas probably won't be underpopulated at any point while trying to complete phase 3, so you'll be just fine.
Once you finish that up, head on back to Ilistar and turn it in! Easy! You'll be rewarded with 11 gold, 99 silver, and 250 Shattered Sun Offensive Reputation. When phase 3 is completed, his quest is replaced with Crush the Dawnblade. The name of the quest is the only thing that changes. Same objectives, same reward. For further information on this quest, you can also check Wowwiki and Wowhead.