Another hint for Challenge 10A
Technically, this is a 2×2 Hill cipher, so while frequency analysis helps a bit, you really need to do bigram analysis for it. That just means counting the frequency of pairs of letters. Challenge 10A starts
RVSUU CNTAN NNFWO ATAEL ILESU NOBCG DDRZS DANDZ
which can be reformatted to
RV SU UC NT AN NN FW OA TA EL IL ES UN OB CG DD RZ SD AN DZ
and in this fragment the most frequent bigram is AN. If you compute them all, across the whole message you will see which ones are the most popular. What do you think they might represent?