Well, Spica is a must-have if you don't have any good archers other than Bashira. Louise is a fairly decent princess if you don't have one. She increases defense and blocks 3 when skill is activated. I would use this chance to get both though. With the increased drop rate and if you have any SCs to spare, do the magic crystal daily H or above difficulty. I think you should get 15-18 if you're lucky.
Drop rate does work with commission sortie I believe.
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Did three PS on my DMM alt. and got the new gold summoner.