It depends of course on the nature of the healing magic and how fast it's working.
The three things I'd try:
Can a spell used to staunch blood flow also be used on a non-bleeding target causing massive blood clots?
Is there magic to increase Insulin production for diabetics? Will put victim in a coma and eventually kill them.
Are there any spells to anesthetize the patient? Is it rapidly working? Can it be quickly and conveniently cast if I don't care whether the patient ever wakes up again?
If the magic "magically" only works on the intended wounds/diseases/maladies then it will be hard to use them offensively. However, if the magic works more or less like modern medicine there are a lot of creative uses that will ultimately kill the victim.