''will have been'' it's future perfect simple>>will+have+past participle
It's used to look ahead to a time in the future when:
>an expected event or activity will be in the past:
''I'll have finished work by five o'clock tonight.''
>a duration of time will be in the past:
''I'll have been here for a year in January.''
''You will have been a great help to me.''( but i think something is missing here, the exactly time when he will have been a great help to you...)
I hope i'll receive the necessary corrections from a teacher.
Student or Learner