To earn the Dressed to Impress achievement/trophy in Gotham Knights, you must “Craft a complete set of Legendary equipment (Suit, Melee Weapon, Ranged Weapon)”. Legendary equipment is denoted by its yellow color, and typically has higher stats and more mod slots and passive stats as compared to more common gear.

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Crafted a legendary suit, melee weapon and ranged weapon to earn the “Dressed to Impress” achievement.

In order to find the blueprints for legendary gear you’ll need to advance the main questline - if you’re early in the game, legendary blueprints simply will not drop. Once you reach Case 06, however, legendary blueprints will finally start to drop, and you can find them from pretty much any source of loot; defeat criminals during opportunistic crimes, premeditated crimes, loot chests, complete challenges, etc.