Image: UbisoftOfficial screenshot from Ubisoft.
A Quick and Reliable Remedy is the story achievement for completing Memory Sequence 4 of Assassin's Creed Syndicate. It unlocks after Sequence 4, which follows the twins' fight against the Templar trade in Starrick's Soothing Syrup.
Play the main memories of Sequence 4 through to the end. Optional objectives do not matter for this one; only the 100% sync achievement, Mentor, asks for those.