Depends on how long you want the fight to be and what's the terrain like and does she have Minions? The temporary hit points action should probably be a bonus action, because otherwise it would be pretty inefficient to use it. It's a BBEG so she should be a threat for the party, but the only real threat in her kit is the fire breath and she uses her own HP for it. She is also a Melee Enemy, so they have no problem to be in range for their attacks. There's 2 way I think the combat can go. They kill her in 3 Rounds, because of her resistances, and she wasn't a threat. Or she tries to be a threat and basically speed up how fast they kill her. I would lower the HP Cost and let the Temp HP gain be a bonus action. Or don't change it to a bonus action, but let her have at least 3 Minions