I can see quite a lot of opportunities. Most of them depend on the assumption that you, as a dedicated Healer, have access to not only generic healing spells that cure minor ailments, but also a wide arsenal of specialized healing spells that address specific body functions and problems, and that these spells actually require you to judge if they're the right tool for the job, and that they will work regardless of the current state of the target.
For instance, you could cause a target to overdose on a certain hormone, such as adrenaline. In that specific case you'd have to be careful to actually make them overdose, as giving them not enough to die might potentially turn them even more powerful. It's quite easy to give "more than safe" amounts, though.
Depending on the biological knowledge behind these spells, you might have access to a defibrillation spell that induces a strong (relative, that is, strong when introduced to nerves) electric current which can be very dangerous when the target is not actually suffering arrythmia. Alternatively, depending on the level of control that you have over the magic, you might be able to induce a current in other nerves, potentially paralyzing your target.
And you might have access to a cauterize spell to sear open wounds -- which can also be just as dangerous when used on healthy skin. 227
You could turn the target's immune system against it by activating responses out-of-context. You could administer narcotics and render them unconscious. The list goes on and on.
However, if you don't have fine control over the magic, your actions are rather limited. If you only have access to spells that cure specific (or generic) ailments, the most obvious solution could be to use healing spells in a far greater scale than they were designed for, potentially causing many of the effects I just listed. Doing so might cause the body to deprive itself of resources, which probably causes a brief surge in power (during which you might want to run away) but causes death by exhaustion shortly after.
All of these things, however, might take a lot of time (which you might not have) and probably drain a lot of energy on your part (which might be better spent on healing allies).
All in all, if you are a healer and your allies are all dead (on the assumption that you actually have allies and that you didn't go solo as healer), the best recourse might be to run. 227