Amaterasu might not be available for Jewels, so don't bank on that happening.The "which Soul should I use?" thread should answer that.Officially nothing is worth using Elixirs on. It's better to be strong enough to clear content, and if you fail, then you fail. Elixirs are not cheap, so don't use them willy-nilly.
Then again, if you don't ever use them, what use are they?Veteran players will tell you that it's Kaisers only. However, that goes with the premise that you have everything that has come out so far. So, if you ask me, it's in order:
1. Character Atk+HP Eidolons. Simply put, for the hardest content, you're going to be using one of these with a P2W Eidolon as assist. As such, these are very much recommended for the hardest content.
2. Elemental Eidolon for each element. These are your basic go-to farm Eidolons, as they provide you with the most damage output whenever you're not looking exclusively for P2W Eidolons.
3. Kaisers. They're really, REALLY good and easily worth the 60k Orbs.