We. The Revolution

This is done in Act 3 when Paris is turned into a battlefield. Each district has a certain population count some people in each district automatically run away from the city at the end of each day as long as the district is owned by you (otherwise they're killed by Bruno's revolution). Also, during any combat, up to around 4500 people in that district have the chance to run away as long as you're able to hold out long enough or win the battle. I recommend recruiting a logistician as soon as possible, as they increase the number of people evacuated at the end of the day in any district they're placed in (If used in combat, they will increase the number of escapees per turn, but it doesn't look like they increase the total number of escapees for that combat). There is also a new family action called "safe path" that says Bernard will lead refugees out of the city, but I am unsure if that has any effect on the gameplay. You can see the total number of escapees vs killed at the top of the screen (escapees on the left, killed on the right). When the escapee number reaches 350,000, the achievement will unlock.

You should try to hold on to as many districts as possible for as long as possible. Focus your defenses around the southernmost district (the one that used to be the enemy HQ). This is the only place they can attack at first. I was able to keep them out almost the entire time, but you probably only need to keep them out of here for a few days. On Day 6, the enemy will open another front in the top right district, so make sure you're prepared for that too (They'll try to sneak around again to the top left on Day 9-10-ish, but if your defense is going well, this shouldn't matter too much). Keep your reputation and influence as high as you can, reputation determines how many recruits you get each day and you'll need influence to build barricades (6 influence) and recruit logisticians and medics (5 influence each) from your HQ. From what I can tell, the barricades only work if the enemy actually owns a district, not when defending from an incursion, so if the district is fully evacuated, it may make sense to pull your defenses back (the enemy army compositions also seemed to differentiate and become easier to deal with after taking over a district but take that with a grain of salt).

My compositions were usually 2 cannons, 5 shooters, and 5 melee or 6 shooters and 6 melee, depending on whether or not I had cannons, but your mileage may vary. Battle tactics may vary depending on a slew of factors, but the battle order is always cannons -> shooters -> melee, and damage dealt is relative to the health of the row, so the best call generally is to knock out their cannons and shooters first, while making the most of yours (Defense or Suppressive Fire tactics are usually the best at this, but again your mileage may vary). Don't forget about your medics either, they can make all the difference in how long a line will last. I'll usually put them in the melee row, but sometimes I'll put them with the gunners if I feel like the melee's going to get immediately eviscerated (typical of aggressive attackers with lots of ranged units).