If you have escalated your concerns to the manager and they were dismissed then that says something about the environment where you work.
I do not know how helpful this is but I will throw some considerations out:
Does the Credit "A" receives have an impact on your growth at the firm? Does A's behaviour have an impact on your ability to get a bonus? on your billable target? If it does and you have shared your concerns and nothing is being done about them then I believe you will slowly start to resent your workplace which will naturally lead to you look for another job.
Are you getting good experience at this workplace or are you are always stuck doing the grunt work? If you are getting good experience, is it enough to justify working at that place?