Spellbane is essential, but it's a once per day kind of spell. Fire and forget, basically. I would prefer to use one Mythic Power each day to cast it with Wild Arcana. That raises the caster level by +2, so right now it would have a minimum duration of 22 hours, but more importantly, the current caster level is 22nd. We want its caster level to be as high as possible to avoid it being Dispelled or suppressed by another Spellbane effect.
It should actually be cast each day while Dany is refreshing her Persistomancy, with Lya using an Harmonic Chorus spell on Viserys to push its caster level to 26th (in conjunction with a canceled Moment of Greatness). With the bonus from one of Viserys' rings, that would raise the caster level of the Spellbane to 30th vs Dispelling and suppression.
I don't mind Fiery Body as one of our 9th level spells, though. There are ways to manage the increased effect from Cold damage.
The other 9th level spell I was thinking about for Viserys is
Effulgent Epuration. There are all too many spells which have majorly deleterious effects even if you make the saving throw or simply don't allow a saving throw at all, some of them we make frequent use of. Avasculate, Stun Ray, Amber Sarcophagus, Maze, Irresistible Dance, Shivering Touch, etc. Spell Resistance applies against almost all of them, but it's way too easy to boost one's ability to overcome SR with a spell or two, feats, or magic items. Way too many spells to protect against all of them with spells like Greater Spell Immunity or Spellbane, that's for sure.
Effulgent Epuration is the perfect answer to those spells. The protection can also be passed on to others, which is really nice when you have a bit of time to buff up prior to a fight.