During the arc "Dangerous Habits," John Constantine convinces the First of the Fallen, who has come for the soul of one of his friends, to drink a beer from a pool under the friend's house. The First finds it to be perfectly good, in fact the perfect brew, until Constantine points out what it's made of - holy water. Set to revert to holy water should the candles near it be disturbed. John proceeds to kick the table with said candles over, burning the First from the inside, then shoves him into the pool, disrupting his physical body long enough for the deal his friend had made to expire.
Two from Dangerous Habits, actually. The aforementioned one, then much later, after tricking the Second and Third of the Fallen into both buying his soul, forcing all three to cure his terminal lung cancer, as his soul going to hell would force all three into a war that none of them could win, and at the very least would leave them open to decimation from heaven. Needless to say, all three are understandably upset. This is compounded when, after having his entire body rebuilt from scratch, John turns around and gives his famous "Up yours"◊.
Mitigated slightly by all three forgetting that "alive" does not equal "healthy" or even "having all limbs and various manly parts". Just chalk it up to the Rule of Cool.
Ah, but here be the rub: he forces the demons' hands by slashing his wrists... and he makes it quite clear that he will kill himself repeatedly until he gets what he wants. It Crosses the Line Twice, but still quite awesome.
Moreover, in this case, "alive" does kind of mean "healthy". The whole point of healing him is that they don't want him to die, because as soon as he does, they'll be forced to go to war over his soul. So, obviously, they don't want that to happen anytime soon... which, essentially, means he's got the three rulers of Hell protecting him for the rest of his life, which just adds to the sheer Magnificent Bastardry of his plan.
This troper's personal favourite is from an early Jamie Delano issue, in which Constantine saves his girlfriend from a four-headed, eight-armed monster which a demon created by fusing four football (=soccer) hooligans together. How does John do it? Seven words: "What do you do on Saturdays, lads?" Whereupon both pairs of heads shout their team's respective names and pummel each other to death.
Also, peeing on the king of the vampires.
Are we forgetting the entire economy that John singlehandedly destroyed in the first few issues? The entire economy of hell?
In the original The Books of Magic run, Timothy is taken to a party packed wall-to-wall with any number of beings from John's own Rogues Gallery who have just discovered who Timothy is, putting him in no insignificant danger. John arrives, opens the front door, calmly lights a cigarette and says "I think you all know who I am" and walks out with Timothy without another word.
Even better is that he also rescued Zatanna, who it is pointed out is much more powerful than John, but that's not the point. Because they're not afraid of Zatanna.
Because they know she's a hero, and won't stoop to the bastardly things John will quite happily do as a first resort.
Chas, John Constantine's most faithful friend, gets one after Kit dumps him in Fear And Loathing. Chas picks up a drunk and disorderly John and even gets punched in the face for his trouble, but John's inner asshole comes to the fore when discussing his problems with Chas and he tells Chas to piss off while insulting Chas's wife. Chas proceeds to BEAT THE LIVING SHIT out of John and dumps his head in the toilet. And the clincher is that John can't do shit about it, since deep down he knows he was wrong and he knows all the shit Chas has to put up with to be his friend.
Speaking of Chas, The Knowledge has him defeat a demon by himself while John is on vacation and start patching his relationship with his wife. It turns out the Knowledge (the perfect, well, knowledge of London's streets required to be a taxi driver) was originally a sealing ritual.
Constantine has been supposedly Out-Gambitted in "All His Engines" and forced to work for the demon Beroul. Worse still, the soul of a little girl hostage has been implanted in the villain's body, preventing Constantine from just killing him. So, instead, he retaliates in epic style: first, he cuts a deal with Mictlan, the Aztec death god that had been demoted to Beroul's dragon; then, he draws the demon out of hiding, past the magical defences that protect him; Mictlan reunites the hostage with her soul, possesses her and bodily erupts out of Beroul, killing him instantly. And for an encore, Constantine then demands that Mictlan release the girl and leave immediately, and when the death god refuses, he threatens to destroy the girl's body (possessor included) with a voodoo doll charmed with a lock of her hair - a threat he apparently makes good on by setting fire to the hair. Mictlan flees in terror.... but the girl is left unharmed: the hair was pure wig.
"Mortification of the Flesh" is revealed to have been one for Constantine in the ending. Basically, Father Grimaldi, a Catholic priest with a nasty habit of fucking prostitutes ends up accidentally letting a succubus into the Vatican; before anyone can find out about it though, he's visited by an angel that explains that somebody will be there to help - and who should turn up but Constantine. Together, they go about exorcising the demon - though they have to borrow a Tome of Eldritch Lore from the Vatican's infamous Black Library, with Constantine temporarily replacing it with a Yellow Pages that he magically diguises as the Tome. One way or another, the demon is finally banished in a very dramatic scene, complete with fire, brimstone, and an apparent redemption on the part of Grimaldi. The two of them go their separate ways, the priest able to return the Tome to the Black Library in peace, secure in the knowledge that his ordeal is over and God is looking out for him... and then, as he goes to put the book back on the shelf, it transforms back into the Yellow Pages. It turns out that Constantine engineered the whole thing with a little help from Ellie the Demon - who played both the succubus and the angel - all in order to get his hands on the Tome. Grimaldi, now left with two copies of the Yellow Pages in place of the priceless spellbook and a very high-ranking witness to his crimes, can only helplessly mutter "Oh shit!"