Is your difficult senior colleague assigning work to you? Perhaps in a way that clashes with the priorities set by your manager? If that's the case you should d

Is your difficult senior colleague assigning work to you? Perhaps in a way that clashes with the priorities set by your manager? If that's the case you should deflect his assignments by saying "more than happy to help, but I need to prioritise this with my manager before working on it." Then discuss relative priorities with you manager. No need for finger pointing or blame storming, and no need to make it personal. Frame the conversation as being all about sorting out competing priorities. If your manager has got a clue he'll see that your senior colleague is introducing dysfunction.