In addition, the wiki page for light Tsukuyomi points out that the element resist debuff also reduces resistance against debuffs from that specific element. So light resist debuff functions both as light-specific def down and a debuff resist down effect for light-only.
So if you don't already have a normal debuff resist debuff, you use the element resist debuff first, then try to throw your other debuffs (from kamihime of the same element).