So I had Renly in play with a duplicate and my opponent played Flame Kissed on him during the marshalling phase. I'm still learning the intricacies of timing of effects, but it seemed to me that even after saving Renly (and thus the attachment too), the attachment would have the same effect on the saved character killing him again. I had though he was pretty well protected until this play opened my eyes. I just wanted to see if I applied that properly.

The "kill at 0" effects are special effects called "terminal effects." And you recognize them exactly as you have recognized it here -- even if you save the character from the effect, it still applies. So yeah, discarding a dupe from Renly isn't going to help you at all. What you need is a save effect that does 2 things at the same time - save the character and make it so that the terminal effect doesn't apply. And, in fact, if your save doesn't do both of those, you aren't allowed to even try to save the character from the terminal effect.

What other effect appart from Risen from the Sean can do that?

It depends on the situation. Remember that removing the card from the terminal state doesn't just mean increasing its strength. You could also remove it from play as part of the save, like with course set for Viserys. You could also combine other different cards, like the plot that makes kneeling characters “cannot be killed” and kneeling to save, like with Maester Wendemyr. You just have to look for the situations.