It depends of course on the nature of the healing magic and how fast it's working. I'll assume the magic is more or less equivalent to modern day medicine and that it can be used without the prerequisite of an existing condition to cure.

The three things I'd try:

- A spell used to staunch blood flow also be used on a non-bleeding target causing massive blood clots.

- Magic to increase Insulin production for diabetics. Will put victim in a coma and eventually kill them.

- Spells used to anesthetize. These are basically sleep spells designed so that the patient does not react to damage and wake up. They must work fairly quickly and once incapacitated the victim will be an easy target for any other attack, magic or otherwise.