What I do for those spells is divide up the dice. D6 damage per even caster level( max 7) fire and d6 damage per odd caster level divine (max 8).
It is not strictly correct, but close enough. Halfbypass would be wrong, you would have to divide the damage instead.
On that note, in pathfinder there is a feat called penetrating strike which bypass 5 points of dr, unless the dr is against all (does not work against dr 5 /- ). Is it possible to implement this? Or is there a way now that I do not see?