While in Badtown, go to agent Willis' house, where you need to go to the basement to talk to him. Go towards the marked area in the north. It is best if you approach it from the west. Once you enter the area that is "oranged out" on your mini-map, you mustn't be spotted. Enter the marked house. Staying inside, you will witness execution of villagers.
Leave the house and eliminate two guards, any way you want.
Now you need to cross a minefield. You can easily discern them in the water (the screenshot). You will also hear a characteristic sound once you get near them. Keep a safe distance and speed, because the mines can also explode when passed at a larger distance.
The problem starts when you enter the area overgrown with grass. You can now get up and watch for a blinking light of the mine (the crosshair in the screenshot), but you should focus now on listening to how far away from the mine you are.
When you find Rongo and free him, follow him closely, while killing pirates. The mines are all over the field, and the agent can deliver you to the other side safely.
Keep eliminating your enemies until you reach the designated house. At this moment you will have to start protecting Rongo who will be busy searching the area for documents. The first wave of your enemies will approach from the direction of the field that you earlier had to cross yourself.
Inside the house, on the crate, (the screenshot) there is quite a lot of ammo, in case you need it. Every wave of the pirates will be marked on your mini-map, thanks to which you will know which direction they are coming from. After you kill the last one of your enemies, Rongo will finally succeed finding the documents. Approach him to complete this mission.