Only one Phantasmal Killer is sold in the store; it's not an infinite store item like most of them are. So grinding money won't be helpful in that respect. Being able to duplicate non-plot items would be really neat, but the only way I know to do that is by using "Export Character" and multiplayer mode, which is both dubious rules-wise and really slow. (Basically, you export your character holding the item inside a multiplayer game, drop the item, save the game, then re-join the multiplayer game using your exported character rather than the character that's already there. This causes the game to reset all plot flags on the character for the chapter, but the way most chapters are structured, it's not the plot flags on the character you care about; it's the inventory and the plot flags on the module.)
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