I don't think we have that much elemental resist down debuffs yet. if i remember correctly, the only ones who can do such a thing as elemental resist down right now are Light Tsukuyomi and Mars (unless im forgetting someone here)
so yes, proserpina is probably the first one we'll have in our version for dark, then the next one is awakened satan
edit: guess i misread your question, i just rechecked what proserpina can do and its a def down A frame 15%. well currently we have nyarlathotep that can do that, but besides her I don't think we have anymore A frame def down dark himes in the SR category. for the SSR category we do have Satan (15%) and Amon Unleashed (25%)