Originally Posted by
sanahtlig
Added DEF debuff massively increases damage. You can use the Advanced Damage Calculator in my Toolbox to directly answer this question. Sadly, the Temporary Effects Damage Calculator still has a bug that mishandles comparisons between groups with different numbers of same-element kamihime, so you can't use that for this usage case. But in general, you lose 15% damage going from 5 to 4 same-element kamihime, while you gain between 1-2% damage for each additional point of defense down. That obviously ignores the benefit of the ATK down, which is also important.
And also, as pointed out above, that's not the only useful skill Amaterasu has.