Blood Alchemist
Jim joined the Alchemists Guild to study herbs and potions he can take instead of normal food to make his waist as tiny as possible, but it only worked to a certain limit. After burning the last layer of fat, he had to face the next barrier...
The severe pressure was crushing his main artery, preventing the normal blood circulation!
Completely by accident Jim found a tome on blood magic, among other things it listed the technique of using air for blood transportation. This technique was used by ancient warriors to prevent blood loss by absorbing the lack from fresh corpses.
Jim studied the book day and night for months, trying out all kinds of daring experiments until he learned the secret of transferring the blood from above his waist to below it, completely bypassing the waist.
Alas, after winning another couple of inches on waist size, Jim bumped into the next barrier, the backbone itself! But we'll save that for another story XD