Not only a healer could do that. A healer specialising in dental spells would kill people with ease.
You cast two spells - one for deciduous teeth and a second one for jaw soft tissue to regenerate in a very fast manner. Or for salivary glands to instantly produce a lot of saliva. Your enemy is now suffocating.
Second approach. You speed up liverthe liver's healing process. Liver regrowThe liver regrows so fast that it's painfullypainful to move and all intensitiesintestines, literally, are bursting out.
Third approach, you. You speed up the work of the kidneys or the stomach. Your enemy is now dehydrated, but also his bladder is full and he really needneeds to go to the loo.
And at last, cardio spells. You're a healer Leonard, goddamit! Cast a higher pressure spell on his heart and tightening vessels spell on, let's say, the legs. You can the play to make him faint or to create an infarct. Or just a plain old heart attack if you can stack high pressure/low pressure spells for 10 seconds.