NeroMasters

Best answer as chosen by Asker / Ratings
Find 2 Pumice pieces one is stolen from Ark and one is stolen from Ozma
Another is found in a Chocograph take the pieces to Hades in Memoria
He will synth them into a pumice which teaches Garnets Final Summon
Ark the Dark Deity I hope this is what you were asking about?

As for Eiko her final Summon is Maiden I think you learn it from
Ribbon accessory if I remember correctly... Which you get in the storyline

I hope this information was helpful

~Nero