
From the dictionary at
http://dictionary.in.gr :
σκληρός εγκληματίας - "tough criminal" (in the sense of "hardened", I suspect)
δύσκολος χαρακτήρας - "difficult person"
I'm not sure there's any ambiguity there, to be honest - "tough criminal" refers to the criminal him/herself, whereas "difficult person" really means "person difficult to get on with" - the adjective refers to the act of getting on with someone, rather than the person themselves. Two different contexts, in which it's reasonable to expect two separate adjectives.
My understanding was that Simon's suggestions above are correct - but then I'm not a native speaker, I may have been getting it wrong for ages!