Hercules or Shingen if you want your soul to keep up with someone like Brahma. I'm not sure if Brahma is fast enough for this, but if you're forced to use a slow soul, you can try putting Brahma in the last slot and let her burst on her own, then wait for her to catch up again for FB. You may or may not be able to FB faster with this setup, but at least Brahma will have her insane vigor buff up more often.
Also, unlock accessory slots and get a tiara set on your bottleneck SSRs asap, those help a lot.
Fire is sort of intended to be pure offence though, at least once you get to the SSR level - there're close to no defense or healer himes available but most are either fast, hits hard or have strong offensive buffs. Fire is not a good fit for an endurance build - yes you can bring SR healers, but fire lacks good ascension weapons. I'm assuming you're fighting rags and is comfortably above 10k hp, so heal without ascension buys you very few extra turns, while using up a hime slot that can be used for more offensive buffs or a heavy hitter. Light or water is generally better if you want endurance although I think thunder is viable, too, since you have Dian.My fire is pure offense team
Unlike me, it sounds like you have a viable grid or can get to a viable grid for multiple elements, in which case it is probably more efficient to use different elements for different content rather than trying shoehorn one element for all situations. For example, use your fire team for wind raids, where elemental advantage and the free potion helps you deal with lack of healer. Use your thunder team for GO where you can stall in the early waves and have Dian make potions for you to use for the final boss.
Before someone calls me out on it, yes, I do shoehorn my light team into literally all game content outside tower (even dummy, I used light for all 6 dummies.) However, that's only viable for me 'cos I invested all mtix and bricks into my light team and in addition to the regular offensive himes, I got myself a bunch of utility himes that no one seems to like (Sol, Eros, Raphy, Atum, I have them all and I use them all.) Without some insane luck, you need to be willing to at least spend on mtix to get that kinda team. Non-light nut should not attempt this.