It seems that a sorta reincarnating Phoenix could work. Give them a interataction that has a very high time-limit in use that temperaraly turns them into a "Pile of Ashs" thats invulnerable the same way a Fluffball is invulnerable. After a few moments, brand-spanken new Phoenix. That bright-light/fire burst they can do would be probably better than just making them realy hot, and a hot dust that causes temperary blindness sounds like a good interaction they can do. The question is, semimegabeast or a simple creature? They seem rare, at least, and there probably too powerful to let them fly around blasting the land with fire.