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.