Looks to me like the Lord of the North Wind has a bit of an advantage over the Creator of Evil Dragonkind... fast healing and a horn that smites? Plus the whole powerful dire charge thing, and more HD to boot. No wonder that for all Tiamat's hatred of Bahamut, she hasn't simply gone and torn his throat out. She isn't simply avoiding a fight that could go either way, she's holding back from getting herself slaughtered. And Bahamut hasn't gone and killed Tiamat mostly because he's merciful, and might still be holding out hope for her...