Just use Karten, he is broken... Dark DMG helps a lot, Karten can triple cast it. I chained Karten with my own Esther with dark element GS. FTKO
If you can't pull Karten 7* day 1.... His most damaging skill is an aoe light attack. Get light resist and you are fine.
If you hit him with anything other than dark or non-elemental he retaliates hard.
Just use Dark weapons, have a provoke tank, and Sylvie. +100% Light resist, and can imbue a good attack chain on bonus units. Just do make sure not to use anything other than Dark or non-elemental, or you'll go squish.