Dusk Diver 2

Story-related and cannot be missed.

Chapter Unlocks: Weapon Rank 8 (required for skills). New items in Tumaz Mart Secret Store (Large-level items).
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This trophy is for completing Chapter 8, "Inquiry for Truth", which also happens to be one of the most important Chapters. This is in part because being in Chapter 8 means you can now upgrade your weapons to Rank 8. Every character has new skills you can buy, and it also unlocks the ability to upgrade your current skills. You'll need to do this at least once for the Diligent Training  trophy.

It's also yet another Chapter that has loads of new side quests to do, including Companion Quests for each party member. Additionally, it's a major turning point in the story. After you complete this Chapter, the remaining Chapters in the game are much more heavily focused on the main story, so most of the things to do on the side will drastically slow down. In fact, except for one other Chapter (Chapter 11), no new side quests will appear after Chapter 8/starting in Chapter 9.
 

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You should absolutely complete ALL remaining Companion Quests before continuing with the main story. At the start of the next Chapter, the game will place all side quests on hold, so this is your LAST chance to complete the missable side quests. If you do not, some of them will automatically fail and you will no longer be able to complete them after this point. Nemea's quests in particular are highly missable.

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Despite this Chapter's importance, the actual story quest is relatively short. You'll visit a couple locations, but the primary location for this Chapter is the Dadaocheng Control Area (Youshanding). After you finish that area, you'll unlock a new location, the Dadaocheng Pier, where you'll fight this Chapter's boss, who just happens to be a familiar face...

BOSS FIGHT: D.D. (AKA "THE MYSTERIOUS WARRIOR") – PART 2
Yep, time to fight this strange Diver once again. There's no real need to go into detail here as it's the same exact fight you faced back in Chapter 3 (see Welcome to this Crazy City ).

Once you defeat her, you'll run back to the Tumaz Mart to discover a horrible surprise, after which you'll earn this trophy.