Only Atlas Rings (Rainbow Diamonds), Ruby for Beserk Ring and Green Beryl for Emerald/Fairy rings are worth crafting.
But I still prefer having Dias join on universe mode and everything will 1 shot Rena as long as Claude isn't in the party yet.
While I did save the file just in case those MP draining enemies don't show up I did fight only those until I could get the skills required to steal the said stolen treasure chest item.
Then saved and reloading until the Marvel Sword can be equipped then I could beat all the other Lasguss Mountains enemies and stayed their for +40 levels and did ALL of that before Celine joins on the way to Cross Cave (but ditched Celine) since I ditched all mages that aren't Rena for all 3 difficulty settings.
Claude Opera Dias and Chisato are my best party members without Dias, Claude Opera Chisato and Bowman are my best party members.
Stealing can be done before Celine joins on the way to Cross Cave its just that only Ernest has them to steal which is more then half way through the first part of the game.
But even with the stolen battle suits I still had every super boss and some required ones do 9999 to 1/all Pc's at once on Universe and still never had guts kick and and pre bonus dungeon accessible I had nobody at levels over 100 either.