In order to take down the most powerful character in the DC Universe, you have to get a little creative. And that's exactly what John Constatine did when he managed to use an illusion to trick Dr. Fate (Nabu) into getting himself killed - a feat that was ridiculously impressive given his powers.
When it comes to magical abilities, John Constantine is no slouch. He's shown to be a great magician, with his knowledge of the occult coming in handy on multiple occasions. However, next to Doctor Fate, there's no competition to who possesses better abilities. Doctor Fate is the most powerful sorcerer in the DC Universe - and probably in all of comics. He wears numerous mystical artifacts, including the Cloak of Destiny, the Amulet of Anubis, and most famously, the Helmet of Fate - and is often guided by the spirit Nabu the Wise. With the relics, Doctor Fate's magical powers seemingly have no limit, which makes him a great ally to the Justice League.
In Constantine: Future's End (2014) by Ray Fawkes, Juan Ferreyra, Tanya Horie, Richard Horie, and Pat Brosseau, the story takes place five years in the future from DC's continuity at the time. Constantine steals the Helmet of Fate and declines Nabu's offer of wearing it and joining him on a "walk the road of all possible futures." However, during an argument, Constantine accidentally picks up the helmet, which leads to him to wear it. He is transferred into his own heart, thanks to the Auditorium of Anubis spell (Fate is suspicious he was able to summon Anubis). In order to break it, Constantine challenges him in a deathmatch by proving "you've ever given a damn about anybody other than yourself." Fate explains he's saved numerous innocent lives and banished great evils. However, Constantine reveals he knows Fate actually summoned numerous people to potentially wear the Helmet of Fate, ultimately leading them to their deaths. Constantine brings up Khalid being stuck in Hell and declares himself the winner. Anubis subsequently picks Fate up and eats him.
Fate was right to be suspicious of Constantine summoning Anubis because it turns out it was all an illusion. Constantine convinced Nabu it was real as he used demons to act as "stage dressing." With Nabu believing he lost the challenge, he essentially accepted his death - when in reality, he didn't lose anything, he just succumbed to Constantine's greatest trick.
Taking down Doctor Fate is a near-impossible task, but Constantine's trickery led Nabu to his own downfall. It's incredibly impressive that John was not only able to execute the plan but that Nabu bought it. Otherwise, he was going to be fighting an almost assuredly losing battle. The moral of the story? Question everything when dealing with John Constantine, even the most powerful sorcerer in the DC Universe.