The hardest sound for us to say is the slavic 'Szcz' as seen in the city name, "Szczecin", or at the end of the word "barszcz" (borscht or beet-root soup).
When I first learned, they said to pronounce it like the sound in 'Danish Charter.
Then the other one people have trouble with is km as in "kminek" (fennel). Think of saying c'mere (come here) in English without a space between the c and m.