Any class can upgrade class that way, but of course it would almost never happen that a E class could beat that many warriors and take their energy. They might get a lucky kill once in a blue moon by someone not paying attention or getting swarmed, but that's about it. The higher the class the much higher amount of energy must be absorbed, but they have a higher chance of getting said energy.

Even a class S would die if it tried to attack the capital on the palm. The city on the heart will also be really tough to attack and the major trade city. Don't even try it, buddy. ;)