1. Airbenders bend air. (just making sure that there aren’t any room for logical fallacies, gtfo trolls) Thus, they should be able to control the flow of oxygen in the air going through another person’s body. Simply put, airbenders should able to control the respiratory systems of the people around them, including the air inside the lungs of their opponents, should the airbender be powerful enough. Also, as shown when Aang creates whirlwinds of varying size and power, it must be plausible that advanced airbenders can bend air to change pressure in the atmosphere. Following that theory, airbenders are capable of creating zones that vastly differ in air pressure. Basically, airbenders would be able to either compress or decompress their opponents. By mastering terrifying differences in air pressure, airbenders could cut through many organic materials with relative ease.
2. Earthbenders bend earth and to an extent, metal (iron). Thus, adept earthbenders should be able to refine the iron ores found in earth and basically shoot the small projectile, similar to a firearm discharge. Further delving into metalbending, the most expert metalbenders should be able to manipulate the iron present in the human body, which by the way, is around 2.5 to 5 grams of iron in adults. These master earthbenders could then, theoretically, draw out the small iron particles in the body. Without iron present in the human body, the iron-deficient biomass would collapse almost immediately. for one example, there would be a total absence of hemoglobin in rbcs, leaving no possible way to transport oxygen molecules throughout the body.
3. Firebenders are a little bit tricky. They are able to draw upon particular set of elements found in the atmosphere and direct them into fire. Upon the formation of fire, flames, or any other violent states of energy, the firebenders are able to manipulate it to their desire. One of the most lethal and interesting form of firebending is bending lightning. As explained by Iroh, this apparent form of lightning is the guided form of two kinds of energy, yin and yang, precisely as the two energies are restoring balance with each other after being separated by the firebender. The most important aspect to point out here is the fact that firebenders can separate or upset the balance of yin and yang, thereby allowing them to direct the two massive forms of energy back together whilst providing the violently colliding energies a pathway to follow. This lightning, if efficiently streamlined and properly guided, could strike its targets with deadly impunity. Basically, firebenders could to an extent, bend electricity. With electricity, waterbending becomes futile, and with an electric current strong and precise enough, these select firebenders could stop the heart or even restart it.
4. Waterbenders bend the three forms of water: solid, liquid, and gas. The complete and total control of the most valuable commodity in the world gives waterbenders great destructive capabilities. First, waterbenders are influenced by the lunar cycle, as the firebenders are influenced by the passing of comets and strength of the sun. Under a full moon, the waterbenders could control the bodies of their opponents, as the human body is around 70% water. However, it is not clear if the aptitude of the waterbender could grant him bloodbending or if the lunar cycle is the only causation of bloodbending, as Katara performs bloodbending when a full moon is not explicitly shown. Second, water exists in all life forms. Water is often called the universal solvent, and it is truly universal. A waterbender, as so long as he is surrounded by living organisms, could never become helpless. Waterbenders could even bend plants that were mainly composed of vacuous tissues holding water. Waterbending’s destructive capabilities range from instantly freezing their opponents in ice to pressurizing small quantities of water to such an extent that it slices through metal. A most creative method to efficiently eliminate a threat through the use of waterbending could be freezing the particles of water found in the opponent’s body. Take note that when water in liquid form changes to water in solid form, the water molecules, unless trapped to conform to a solid shape, will slowly condense themselves in such a manner that results in a crystalline solid. Even if this effect was to only affect the bloodstream, the consequences would be nothing short of devastating. Millions of red blood cells would rupture simultaneously as the cell membranes are punctured by the crystalline shards of ice, and the blood vessels would resemble swiss cheese, which is a grossly under-qualified comparison.
(I’ve missed couple of things I know, but I’m too tired and sleepy to address all the missing factors right now.)