Full Synchronisation Requirement - Do not lose more than 40 rioters.
Simply bribe each of the heralds to gather up a large crowd of rioters. When you’ve paid each of the heralds, head to the objective marker to begin the fight. Be careful beforehand though to not walk in the small restricted area in the middle of the plaza and scare away potential rioters with early conflict.
During the fight, you need to call in your Assassins as soon as possible. Try to only use one Signal in the first phase and avoid losing more than 4 or 5 rioters or it will be difficult to stay above 40 for the optional objective requirement. When more pitchfork wielding rioters arrive, it’s time to call in the rest of your Assassins. Then, use poison to slow the enemy down a little, and ranged bombs to stun them to help the rioters make it through alive. Try not to get caught fighting one particular guard or group and letting the others slaughter your allies. After the gate comes down, watch the cutscene and be ready for the next fight.