Tsukuyomi (SR Dark) or Moonlight Maiden Tsukuyomi (SSR Light; sometimes called light Tsuku or LT for short.) Big difference, one sucks and the other one is awesome. I am assuming you pulled LT.
My understanding is that you need some really good luck to be able to put together a good thunder team without mtix, so, it might be a while.
Sol + LT + Diana is actually already a viable light team btw. Once you have D'Art, EX SS. SS + LT + Diana = hits def down cap. LT's blind + Sol heal + Sol atk down + SS = stay at high health most of the time. It's not a super high dmg team, so fights will be slow, but you should at least be able to clear content with that team. You won't be able to deal with debuff resistant stuff like wind rag, but that's where your fire team comes in right?
Durga is much stronger actually. Just be careful with her 2nd ability - rampaging characters cannot burst or use abilities, so only cast it if you know for sure that no one needs to burst or use abilities for 2 turns.