Assassin's Creed Origins

"The Greater Good" is the final main story quest. There are a total of 6 main story quests in the DLC. The first three are detailed in the "Surgical Strikes" achievement solution. The final three are detailed below.

No Chains Too Thick

Head to the marker at the Hidden Ones bureau. After all of the cutscenes, you'll be outside an encampment and tasked with rescuing two people inside. Use Senu to mark them both. There are tons of enemies inside, so best to approach stealthily. One target has to be escorted out to the objective marker, the other has to be carried out to the objective marker. Place the woman you're carrying onto a horse and ride off with her away from the encampment to complete the mission.

Sic Semper Tyrannis

Head to the objective marker to talk to Amunet. Hop onto a horse and follow her. You'll come to a village on fire and under attack by Romans. You've got to kill them all. You could make this very hard on yourself by hopping off your horse and trying to assassinate or fight them all. The easy way is to just ride around on your horse and kill them all. If you get knocked off, kill the guy and hop back on.

After you've finished them off, go talk to Amunet again. She'll tell you about your next target - Rufio. He's on a boat off the coast, and his boat is surrounded by other boats. I recommend taking out enemies on the surrounding ships because if you get spotted they will be firing mounted crossbows at you from both sides and it's a massive pain.

Once Rufio is dead, confirm the kill to end the mission.

The Greater Good

Head to the marker and talk to Amunet. Follow her to the rebel camp to look for your target. Head up the hill and talk to the guy outside the hut to find out your target is at the nearby sunken ship. Head to his location and up onto the scaffolding to trigger a cutscene and a boss fight.

This boss fight is pretty annoying but it is made much easier with the skill that starts your combat with a full adrenaline meter. How the fight works is you'll square off against him on a platform. He will continually try to hit you with his shield to knock you off the platform. If you can, hit him with your special attack and try to last as long as possible on the platform.

You'll unavoidably get knocked off at some point. When this happens, swim to the nearby scaffold and climb up it to the mounted crossbow. Hop on and shoot him. He'll release his crossbow and run up to your platform. Once again, hit him with your special if you've got it and try to last as long as possible before he knocks you off.

Repeat this process over and over until you beat him. Watch the final cutscene of the DLC and the mission will complete, unlocking the achievement.