The human body is a complicated set of interacting systems, with most of them needed for everyday motions, and especially needed for combat. Interrupt any one of these, and you can take a person out of a fight! Choose one system, alter a process very slightly, and watch the person succumb to their new ailment. This is especially true for the following systems: circulatory, nervous, skeletal-muscle, and respiratory.
You're going to need a good definition for efficiency in killing. Do you mean quick or painless? Do you simply mean the method of killing which expends the least energy? I'm going to an amalgamation of all three. Not to compete with the list which bowlturner started, I will simply add to it:
Prevent Neurons from firing. Seal up the chemoreceptors on neurons, or simply deform them, and even your most hard-boiled adversary is reduced to a vegetable or even just mere warm matter.
Turn muscle fibers into the cooked version of themselves. This works especially well for the heart.
These are specifically non-lethal, but will still harm individuals.
Activate (or disable) the voltage dependent calcium gates in people's cells. This will cause their muscles to tense up, rendering them useless until their calcium gates restore equilibrium. The alternative is just to make their nerves fire randomly or all at once.
Change their eyes to be super sensitive to visible light. It effectively gives a flash-bang type effect. Yes, this is technically an enhancement, but few people will be well trained enough to adjust. When people first get glasses, for example, they often have trouble with visual tasks for a day or so until their eyes adjust. (I know I had trouble.)
Stiffen the or remove hairs of the inner ear. This denies your opponent their senses of hearing and balance, but can be undone with regeneration. More on that below.
Snakes see in infrared. Bees and birds see in UV. It can be done, and nature does it for different creatures. You just need to add the correct receptors.
Yes, setting things on fire can be done. You simply need to hit ignition temperatures for that thing. Spontaneously and only be modifying current cells? I doubt that. You could increase the surface area of the items to lower the heat needed to burn that thing. I suppose you could force the cells to produce something that burns when in contact with air, but that is super dangerous!
The best way for a creature to non-destructively produce light via modification of an organism is bioluminescence. Of course, if your magic is in the business of modifying eyes, why not just give people tapetum lucida? Their eyes will shine like a cat and can see much better in the dark; certainly much better than if they shine like a beacon.
Can skin be made fireproof? If you could convince the skin to exude a mucus or some other non-flammable substance, this can be done. It should be noted, however, that everything burns, just not at the same temperature. Anyways, a non-flammable mucus can have the benefit of providing a layer of insulation between the skin and the flame. I suppose you could have thick scales on your skin, but then that really isn't skin, is it?
Can magic be used to heal better? I do not know; it is your magic. If this magic can only change people's biology, then yes, it can help heal. For instance, if you increase the number of or stimulate the production of stem cells, regeneration becomes doable. Is it faster than normal healing? No. It still takes a while, but the benefit is that it happens, instead of remaining a dream. The creature regenerating that limb still needs energy to regenerate; it means you would need to eat a lot of food while regrowing a limb.