Just as a back-of-the-envelope calculation, to raise a living body's temperature to the combustion point (let's say, 300 Celsius), would require first boiling away most of the water in its flesh, and then heating the remains by another 200 C.
Loosely speaking, there is about a 60 C difference between human body temperature and boiling water. Additionally, it takes 540 calories per gram to boil water. Thus, just boiling a human dry (required to heat his flesh above 100 C and ignite him uniformly) would require 600 calories per gram.
For an 80 kg man, this is about TWO HUNDRED MEGAJOULES, or about fifty-six kilowatt hours. For comparison, a .38 Special handgun bullet masses about 10 grams and travels at 250 m/s, striking a target with about 3.2 kilojoues. In other words, incinerating a man requires energy comparable to tens of thousands of handgun bullets.
In sum, it takes a LOT of power to rapidly combust somebody--more than a thousand times greater than it does to kill him if he is unarmored.