If they run out of internal energy to use the energy shield, then they will resort to expending life energy to reinforce their flesh with life force so as to resist the heat.
This is a last resort because reinforcing the flesh with life force will only reduce the damage by a small amount if at all.
It is more likely that hardening the skin and flesh won't eliminate the damage caused by fire. It also won't eliminate the pain of being burned alive.
Besides, they need the life energy to enhance their physical power so that they can withstand the pressure and move. If they also used their life energy to defend themselves against the heat, they would quickly run out of it and be unable to continue.
