The Heavenly Convergence technique applied tremendous pressure on the target. Gay-Lussac's Law required immense pressure for immense heat; thus, it was the perfect technique to use.
Rui was immensely satisfied with the theoretical framework that he had created for the Muspelheim domain. The domain involved three forms of heating at the moment: 1) passive convection, which would happen naturally after exhaling super-heated breath; 2) active pressure-heating that required active effort and conscious control; and 3) friction-heating through earth-bending if he was in contact with the ground.
With these two elements, Rui was quite confident that the theoretical framework of the Muspelheim domain technique was complete. While he would have loved to add a fourth element to the technique, it was beyond his capacity to handle at the moment.
