In reference to where you mentioned "darkness = immunity, light = none", Saber would definitely not be immune since she's quite holy/light aligned. Due to Saber's regeneration and magical resistance, he'd really have to be throwing out a flame to do much damage to her. (You say third-degree burns, but I'm thinking Saber would have first-degree burns that would take a day or so to heal.) I do know Ao no Exorcist, and I'm pretty certain that Rin is strong enough even with Saber's magical resistance to do something to her. I'm always open for negotiating. Let me know if you have any thoughts.