Once you start getting quests from General Chafra in the Nubian Expedition, keep doing her quests until you reach the "Fall of Nubia" (after the quest "Wait for It").
The entire chain, Fall of Nubia included, are all simple and straightforward. The Fall of Nubia is actually the only one that might really require any effort, although not a whole lot depending on your strategy. I started off by launching an immediate assault on my enemies with some Barracks units while my city built up and eventually got some back up from my allies around the time I was attacking the last Town Center (out of 4~). It's a big map with the enemy pretty spread out so the only real difficult part may be finding that one last guy you need to kill to finish the quest.