The Long Cut scene from John Woo's Hard Boiled. When John Woo helped create cinematic 'Gun Fu' while working in Hong Kong cinema with legendary films such as A Better Tomorrow, The Killer, and the aforementioned Hard Boiled.
Keep in mind the above scene was only one long cut in a truly epic action finale which took place in a Hospital and was only the last of several impressive action set pieces that took place in the film including bloody shootouts taking place in a Tea House, a dockyard and a Drug Warehouse.
Also the final shootout from Brian DePalma's Scarface. Also suitably epic.
And for something completely different... but still Gun Fu ish enough. From the animated film Resident Evil: Vendetta where Captain America Christ Redfield takes on a bunch of zombies and then is helped by the franchises very own boyish Zelda equivalent, Leon Kennedy, showing up on his motorcycle to help.
Chris Redfield later had an even more ridiculous Gun-Fu fight with the main villain of the film as well later on.
And for a modern evolution of the technique that isn't John Wick, the first scene from Extraction featuring Chris Hemsworth.