"He did not like you."
Translation:On cię nie lubił.
Valid enough to accept it, but as it usually is with putting 'ciebie' at the end, such emphasis is not common.
On cię nie lubił => Kogo (Czego) on nie lubił? (Genitive - everything is fine)
On ci nie lubił => Komu (Czemu) on nie lubił? (Dative) -!ERROR - this doesn't make any sense.
When you have doubts try to ask a question, if the question makes sense then you are right (in most cases).
Seems a bit less natural to me (stronger stress on "you"), but good enough. Added.