HDE 226868 already asked about converting enemies to a liquid state, so I will ask the next step, vaporizing them? It is basically the same as his question, but I will repeat it here:
So, how can I vaporize HDE 226868 my enemies?
This weapon exists. It's called a Rocket Propelled Grenade... with a Thermobaric Warhead.
This is also called a fuel-air explosive (FAE). According to the CIA:
...the effect of an FAE explosion within confined spaces is immense. Those near the ignition point are obliterated. Those at the fringe are likely to suffer many internal, and thus invisible injuries, including burst eardrums and crushed inner ear organs, severe concussions, ruptured lungs and internal organs, and possibly blindness.
Obliterated sounds like what we're after.
The concussive force of the explosion will liquify HDE 226868 the target and the following intense heat will vaporize the liquid target.
As pointed out, you really need to apply enough energy to turn the water in the target to vapour (176Mj). Since this would require a lot of power and generate a lot of waste heat, maybe we should drop the handheld requirement. Once you do that, all kinds of exciting possibilities open.
The US military is openly developing laser weapons in the 100Kw range for use on vehicles, ships and aircraft, and these are in the advanced prototype stage. 100Kw might not sound like a lot, bout this is considered the threshold for a viable military system capable of destroying missiles, artillery shells and mortar bombs in flight. Scaling up means a bigger generator, and a larger or more efficient heat rejection system. Add the need for sensors to accurately detect and track the target and the desire to get to the action quickly, and we get a 747 sized system. There are designs for megawatt class Free Electron Lasers sized to fit the cargo compartment of the aircraft, and the cancelled Airborn Laser Lab (ALL) had sensors capable of tracking IRBM's, so we have an idea of what is really possible.
A megawatt class laser aboard an aircraft has the ability to "see" a vast area 9depending on altitude), and once the target is identified, it can be struck at the speed of light. A FEL has the advantage of being able to "dial in" the frequency of the beam, so you can adjust so the beam will pass through the atmosphere with minimum absorption. And of course, a system like that could rapidly acquire , track and vaporize quite a few people in a very short period of time (and deal with many possible countermeasures they might deploy against the laser weapon).
So the "Handheld" part to the system is the radio in the ground observers hand.
Something a little different: Microwaves!
Your average North American weighs about 80 kg, which is mostly water. The specific heat of vaporization for water is about 2.2 kj/gram, or about 176 MJ to vaporize a person if they're 100% water. According to this lovely chart, we get the best performance for liquid water versus gaseous water in the infrared window between 8-14 microns(near-IR radiation). They're not very large or powerful but I'm sure you could scale them up easily. The radiation is efficiently absorbed by liquid water but inefficiently absorbed by their boiling flesh, so it should pass through the expanding gas quickly and get right back to heating the rest of their body. Bones and connective tissue might be a problem, but you did specify 'most of the enemy'.
The chief problem is cooling, but since you mentioned you could put a tank of LN2 nearby and not have to carry it, this technically counts as handheld. You might also want to plug this into mains power, you will pull down 3MW or more for the whole minute, or about 4 thousand horsepower. A small nuclear reactor could power enough lasers to melt five people a minute, but a single really big diesel generator will work great for just one person at a time.
