Many students struggle with doing curly arrow mechanisms (correctly). Look up mechanisms for nucleophilic/electrophilic addition/substitution (the four you'd do at A-level, i.e. na, ns, ea, es).
The other thing most students struggle with is learning the reagents and conditions for all the various reactions studied.
The weaker students tend to find naming of organic chemicals with multiple functional groups hard, e.g. the proper name for lactic acid.