I think I've mentioned this somewhere else, but my theory is that the fire effects and the damage itself are from different sources, the former being from flammable components (those made from plastic, silicon [I think], etc.), the latter from the metal of the robot actually melting (rather than burning). Assuming the "laser sword" is formed from magnetically suspended, superheated plasma, I think it makes sense for it to transfer a massive amount of heat to the robot on impact, inflicting the damage and fire effects.