Here's what I'm thinking. The Fire Shield is certainly useful, but it's not certain that Livia can be resurrected after a battle if she's killed by Lyde. And it's not certain that the Fire Shield will work on Lyde. However, considering what we've seen, we probably don't have a choice – we need to try it. The advantage is that the weaklings definitely won't be able to attack Livia, and she can continue to deal damage with her magic.
Where do we end up when we save the world?