After Episode 7 you can go back to Keos and talk to Xenia and she'll task you with saving her brother. Then a series of fetch quests will unlock (three in total, one by one), The last quest involves you getting a treasure from Olouros Fortress. If you haven't already, use this opportunity to get your bounty to max and you can easily kill five during this period.
Once you return the treasure to Xenia, the trophy unlocks.
Note: It's worth synching as many Viewpoints as possible as this helps Ikaros in finding the item quickly if you want to do the mission stealthily.