bearded devil- immune
orthon-immune
horned devil-immune
pit fiend-immune
hellhound-immune
tieflings- ?immune (unsure, as ive never really tried fire on them, do know that they get 0s in lowbie quests...)
and the first two comprise of pretty much all high end enemies :/, too much stuff is immune to fire for it to be really widespread D:, would be sweet vs. lailat though.