So what happened to me is that I wiped out Arzagos's hideout and I dueled the boss. I didn't get the last part of the banner. I then found a group of 3 raiders (even on the map it said Raiders, just like Looters etc.), I killed them and got the last part of the banner. I guess, if you choose to team fight them, you should get the last part after you wipe out the hideout.