Visit Free Heap and talk to Petra, a weapon maker.
Recover Weapon Parts
Once you talk to Petra, a weapon maker, she asks you to acquire materials that she needs to create Oseram Cannon, a special heavy weapon. Then you head to bandit clan's place. You will climb up the rocks in the screenshot above.
While you are beating bandits, you collect power cells. To get the third power cell, you will climb the rock wall in the screenshot above. Then, you also look for weapon parts around Scrappers' nest.
Defeat Bandits using Oseram Cannon
Petra crafts Oseram Cannon for you to defeat bandit clans that attack Free Heap once you give her collected parts. Enjoy the special heavy weapon!