En masse and without warning (3,2,4)
I believe the answer is:
all at once
I believe this clue is a double definition.
'en masse' is the first definition.
(thesaurus)
'without warning' is the second definition.
(thesaurus)
'and' acts as a link.
(Other definitions for all at once that I've seen before include "Together" , "Without warning" , "Simultaneously - immediately" , "Suddenly" , "How daffodils appeared to Wordsworth" .)