Imo, B!Ephraim works best with four turn specials like ignis or dragon fang. If you can't get those though, he can be built with galeforce, or if you don't like/have that, I'd keep draconic aura just because it's free.
Galeforce is mostly useful for some surprise defense team wins. Otherwise Ignis is best for damage output, but Dragon Fang usually does just as well because his attack is so high. Ignis has a slight advantage up against units with massive defense stats.
Eh just keep Draconic Aura. It'd be a waste to give him Bonfire or Luna and then later end up giving him Ignis, Aether, or Galeforce. So I'd say just keep Draconic Aura and then give him Ignis as soon as you get fodder for it.
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