After speaking with Anthousa, you should begin weakening the Monger’s hold on Korinthia by burning all of the goods in his warehouse. The warehouse you seek can be found at the port of Lechaion in north eastern Korinth. When you reach the warehouse, use Ikaros to scout and identify any guards. There should only be two of them; one at the entrance to the dockyard and another at the entrance to the warehouse. You can then move in and assassinate them.
Note: Before you go to the supplies and burn them, rescue the two prisoners inside the warehouse, one on the ground floor and one on the first.
When you have rescued the prisoners you can proceed with the burning of the marked supplies. As you burn the goods more guards will enter the warehouse. You can take them out with precise shots or assassinations. When you have burnt all five of the supply stores a cut scene will initiate, after which you will be fighting more bandits alongside a mysterious Spartan soldier named Brasidas. The fight ahead is easy enough, especially with his help; focus on one or two bandits while he tanks the others. When they are dead, speak with him to complete the quest.