Thunder is not an element that works well with heals. It's best to just lube up and go in hard. If you're having trouble, it's most likely because of your Grid.

I'd recommend Dartagnan with her HP weapon for the Mammon synergy. That also allows you to not use Tyr - Tyr is decent, but until her Awakening rework, she's not necessary. For you, Mammon is the only necessary Hime... along with Athena if you fight Water. And Brahma once her Awakening comes along. Raiko is also bloody amazing - her skillset just suits Thunder perfectly. Zeal is by far her strongest skill, don't mistake Raiko for a purely defensive Hime.

With all those SSRs Astraea might not make the cut, but bringing her is an option to make Dartagnan's debuff reliable.