Eh, we'll tone that done a bit, let's say that while Tibbers is structurally fine, she uses very little Aura in relation to regen. But if parts of him are destroyed and she has to make Aura to compensate, she can run out quickly. And also, if she's emotionally distressed, Tibbers isn't focused enough to use Aura efficiently and will burn through her Aura in a burst of energy. Or something like that. :P