Is the moonlight sword good at lvl 80-100? What stats should I put to get the best out of it? I was thinking about 20str and 40-50 arcane, but I don't know how this weapon scales.
It is not good at low levels because you need a lot of stat investment before it surpasses LHB or Chikage. Scaling is B/C/A and it has build in arcane damage so you can't go full arcane with it.
You can use it as a pure physical weapon with 50/25 (bad) or 50/50 (better) STR/SKL and physical gems. After that you should reach 30+ arcane and replace the physical with nourishing gems. At 50/50/50 it will easily outperform LHB.