Mordred's Black Propaganda is her strong point. It gives the boss one extra bubble so that you'll get less Overdrives. Sniper Shot is very good to have from Dartagnan, but Black Propaganda is vital for almost all solo content.

DO NOT buy Andromeda until you have Gawain, Mordred, Cass and Joan. Andromeda is a huge newbie trap. The res is BAD, sadly. It heals barely any HP, and the resurrected teammate goes to the backrow so you can't heal her. And abilities don't go off cooldown while in the back... so once she comes back into the fight, she'll be almost dead and has no abilities ready. Cass does (almost) everything Andromeda does PLUS Dispel. If you really, really want heals so badly, then get her after you've gotten the other basic Souls.

Ea is not particularly good, but SSRs are always SSRs. She has no debuffs, so she doesn't fit into other teams either.

And, again, if you want advice on how to main Thunder, you should read Sanahtlig's guide. It's not a very good element to main, honestly.