After saving Stagg on his airships, you two will talk, at the end of the conversation, you will see Joker instead of Stagg. The militiamen you took out will start to rise as if they have never been hit in the first place, then they turn into Joker. As more Jokers appear, the real Joker will jump onto you, repeatedly press
to struggle out of his grasp. After doing so a cutscene will begin of Stagg being taken off to the second airship, giving you this simple and unmissable trophy.
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Credit: Extreme Royale