I know your pain I was stuck there for awhile with all ranged. I ended up grinding my 5 star, and a few 4 stars to almost their max level on 2-7 to get past it. Also got lucky cause 2 of my allies that had 6* monsters at 120 were on at the same time.
Suggestion would be to dump a few inactive allies if you don't have room then try, and recruit some people that have 6* melee units like sofia or Reinhard as your team mate.
20-1 IMO is the hardest after it I found them easy.