My guess is it would fall under necromancy. The only time we have seen it done is when Harry's magic was stolen, and when he took it back. So to actually take a wizards magic would probably require someone to leave their own body, consume the magic, and return to their body. Harry didn't use necromancy but he didn't go after a person either, just a spirit. To go after a person might require going further into the realm of death which can only happen if necromancy is involved I think. At least that's how I think it could happen.
If it is possible for one wizard to do it to another wizard it would have to be pretty risk, and dark magic. If it was easy to do someone like Kemmler would have just targeted wizards to consume their magic. He would grow greatly in strength, and remove threats to himself all at the same time. Cowl for example could just target all members of the Senior Council... It hasn't happened though. So either not possible for a mortal wizard to do, or highly risky stuff.
The risk would probably be my idea of leaving ones body... While the spirit is out of the body it is probably at great risk of either being contained, resulting in permanent death, or while outside of the body, the body is defenseless, and open to attack.