The Crossing

You have to defeat the mutated Wayne to secure the area. There will be walkers that will join the fray, making it a bit difficult to move around. There are hot steam valves that you can turn to burn whoever is underneath the steam. You can use this to soften up Wayne. If you have molotovs and other grenades, you can use these as well to deal damage. Fortunately, Wayne is really slow so you should have more than enough time to stay away from him.

After killing Wayne, regroup with your teammates and talk to the colonel. Resupply as needed. Enter the warehouse and exit to the docks. Head to the beach first and be prepared for an attack of a special boss zombie. This is one of the two special zombies you can kill in this area for an achievement.

Follow the beach and enter the beach house to find a blueprint.

Head to the next area but don't enter the gate yet. Follow the path straight to the left (east) to find a mod on the table at the edge of the dock.

Go around and follow the path until your quest is updated. Before heading to the prisoners, plant some mines in front of the Workshop building and make your way to the back of the sign. Climb up to find some supplies and a sniper rifle above. This will make the second twin zombie to burst out of the workshop and get killed by the mines you planted. The corresponding achievement should unlock as well.

Head to The Ferry station and talk to the prisoner. You'll enter into group rage so use it to quickly murder all the prisoners.

Break down the control room door and talk to Marvin to complete the quest.