Could a magnetic field be strong enough to damage the body or erase the brain?
Blood is slightly diamagnetic, so if the field was strong enough, could it repel a body evenly?
Could a diamagnetic material go through electrolysis with water then ingested to give the body more diamagnetic properties?
A strong magnet can repel a body, but the effect is due to small diamagnetism of every atom.
You can see a picture of a levitating frog and a video here. And a very good explanation and more videos here.
While mammalian blood is characterized by diamagnetism (as demonstrated here), as far as I know, the human body as a whole is more or less electrically neutral—that is, with all of its summed components the negative and positive charges balance each other out or there are extremely insignificant differences.
If you want to get strict though, it's theoretically possible to use a super strong magnet to repel or attract someone's entire body to a meaningful degree as long as it's not electrically neutral at a given point in time. But even in the video linked above, the guy is using a super strong magnet to observe its repulsive effects on blood and the blood is just barely moving away; definitely not forceful enough to where someone wouldn't be able to resist moving with just their body weight alone.
So to answer your question: can someone be repulsed or attracted by a magnet practically? No. Can someone be repulsed or attracted by a magnet theoretically? Yes.
