All For One is basically the strongest evil person from that world. In his prime, All Might was the only one who was strong enough to take him on, and he did defeat All For One, but the first time he suffered severe inner organ damage. The second time, it was so hard on his body that he lost all of his power.

[She figures it's okay to share this, since he'd been willing to share it with all Guardians, deputies, and allies. As somemone who is not only a deputy guardian, but who also lives in the same building as people from that world, Bulla feels it's information Allen should know. His odds of getting caught up in this on some level are high.]

Knowing if it's possible for yours is important, yeah. You should talk to Mr. Aizawa. That's how we're testing. The people whose powers he can stop with his quirk need to be more careful. Mr. Aizawa was able to stop Remilia from flying, for example.