Thunder seriously lacks debuffs, which this game revolves around. This makes building Thunder teams extremely difficult.

As a general "rule," it is recommended that you keep rerolling until you get 2 SSR Hime of the same element. Or maybe one hard-carry like Sol or Amaterasu.

Out of the current SSR Eidolons, none are worthy of anything. You can just ignore those. Next update will bring us Belial, who is better than a SSR Hime though.