Attacking the man, representing him a demon (2,7)
I believe the answer is:
ad hominem
'attacking the man' is the definition.
(an ad hominem argument attacks an individual)
'representing him a demon' is the wordplay.
'representing' is an anagram indicator (letters must be re-presented in a new way).
'him'+'a'+'demon'='himademon'
'himademon' with letters rearranged gives 'AD HOMINEM'.
(Other definitions for ad hominem that I've seen before include "to man personally" , "(Of an argument) personal rather than objective (Latin)" , "Latin phrase for argument that attacks opponent rather than debates the issue" , "Directed at particular person" , "Relating to particular individual" .)