It might behoove you to delay the encounter by a couple levels. The CR of a half-fiend fire giant is I think underrated at 13. If you use the spell resistance benchmark (typically monsters have SR equal to their CR + 8) the fire giant would come in at a CR 17, which is too high, but CR 14-15 looks about right to me, especially if you're giving it any equipment that takes it beyond the baseline fire giant level. Remember that unusual stats and equipment boost CR (for example, just giving it the 'elite' stat array raises the CR by 1.) To put it another way, I see this encounter as likely to be on par or harder than an encounter with a nalfeshnee (CR 14).
I think I would go with 15 as my benchmark for the 'true' CR (if you've given it Big Evil Leader equipment) as it matches nicely with the caster level of the blasphemy.