The following contains spoilers for My Hero Academia Chapter 354, "Oopsie Daisy," by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
Since his introduction in My Hero Academia, All For One has commanded an air of invincibility. Given his uncountable number of Quirks and unwavering self-assured disposition, it's always been hard to imagine the villain mastermind's vulnerability. However, thanks to Hawks' keen-eyed observation -- and the gift the Hero Killer brought with him from Tartarus -- the Pro-Heroes have managed to identify All For One's elusive weakness at last. Although the sinister villain is still plenty powerful, he's long past his days of prime health.
Perhaps his old age, or the sheer weight of Quirks he's accumulated, has left him unable to survive without a literal lifeline. He's always been shown hooked up to a life support machine, and Hawks noticed he'd taken special care to ensure his medical equipment was still attached to his person all throughout his escape from Tartarus. This strange behavior, and All For One's relative inactivity, led Hawks to deduce that he could only survive on life support.
As he'd never been seen without his bulky headpiece, the heroes further guessed that the life-saving equipment All For One needed was built into his helmet. This realization might have made their task rather simple -- if the helmet hadn't already shown some serious feats of durability. All For One wore it all through his and All Might's slugfest at Kamino, and it only broke after receiving the strongest ever One For All punch shown in MHA. Getting the mask off wouldn't be as easy as it seemed.
There was also the added danger of All For One himself. Swarming him in an effort to take the mask off could easily have turned into a buffet of Quirks for him, so he was paired with "the maximum amount of power in the smallest group possible." The team of top two heroes, Endeavor and Hawks were to exclusively handle All For One -- and remove his mask by any means necessary. It was a task they seemed up to, until All For One began his psychological warfare.
Unfortunately for Endeavor, All For One knew exactly the right subjects that would unsettle him. He'd been berating the Pro-Hero for having Shoto face Dabi in his stead throughout their fight, and finally succeeded in distracting Endeavor when he spoke about the night Dabi burned. All For One revealed that he was the one who saved Dabi from the flames, stunning Endeavor momentarily and allowing the villain to get a particularly nasty hit in. Despite the fight with All For One starting off on the wrong foot, all hope might not be lost yet.
Just as the villain began to charge up his pulse attack intending to finish off Hawks once and for all, Kyoka Jiro showed up floating atop her 1-A classmate Tokoyami Fumikage. Using her Heartbeat Wall, she canceled out All For One's pulse attack and saved Hawks for the time being. Jiro and Tokoyami looked prepared to face My Hero Academia's main villain head-on, and who knows -- perhaps Jiro's sonic attacks will be just what they need to break All For One's mask.