Charles Helliwell's Answer:
You've heard the expression "You can please most of the people most of the time; but none of the people, all of the time". This is an example of working with a team of seven Adults and one Child, so don't despair, because the situations you describe are not of your making.
You appear to be trying too hard to please someone who is never going to be pleased; no matter what you do and how hard you try. So stop right now trying to become the perfect assistant to everybody, because ultimately, your ability to work successfully with the others will suffer and that will reflect poorly on you.
Firstly, understand and accept that this particular individual is not your friend and does not have your interests at heart; he is only interested in himself. Once you have accepted that, it become much easier for you to deal with him.
He is one of eight with whom you work, and the only reason he is able to get under your skin is because you allow him to do so. Consequently, with immediate effect, start treating him as just one of eight. You don't have to like him and you don't even have to please him, if his workload is greater than an eight of the total which comes from the group. After all, you are responsible for all eight of them and not just one and you wouldn't want to tamper with the dynamics of the group by being seen to favour one over the others.
So, for example, when he publicly criticises you in front of the group, return the challenge by suggesting that since the other seven appear satisfied, it's perhaps his briefing or tasking of the work he wants done which may be at fault; and not your ability to deliver it. He won't question you publicly again in front of the group in a hurry; however, don't crow or become too smug at such a public humiliation, because you want the other group members to support you and no one likes a smart alec. Make this more of a rebuke than a humiliation and it's unlikely to happen again.
When he comes to your desk to complain about this client or that, it's not your problem. It's his. So unless you have been directly responsible for a client complaint, refer him to one of his peers, who you know to have good client-handling skills. For example, say to him, 'Speak to Bob. He seems to get on very well with his clients. He may be better placed to advise you than I am'. As the assistant to the group, you should a very good idea of who is good at what.
When he gets pissy and rude because you're busy, just remind him that you wouldn't want to be seen favouring any one member of the group over the others. He'll probably challenge you directly and say something like, 'Don't you like me anymore?' To which you reply that liking or disliking him wasn't an integral part of the job spec when you took the job.
Remember that we never leave all our personal baggage at the door when we clock in to work. Life is just not that simple; so don't adopt an artificial persona, if life outside the office is troubling you. Seek out one of the eight whom you believe has the strongest interpersonal skills and confide in him or her if you want to. Just the simple expression, 'May I ask your advice about something', is the open-sesame to everyone's heart.
Don't get drawn in to any conversations with this individual other than those which are work-related. If he tries to engage with you on a personal level, just say to him 'I'm not sure that I can help you about this; try speaking to one of your peers and colleagues'.
Never allow yourself to get into a one-to-one with him, without asking one of your colleagues or one his peers to join you. When that is challenged, exercise your prerogative of trust. Tell him and them that given his history to date, you have no basis or foundation to trust him or his word and make it clear that it only applies to him and not to the other seven.
Treat the other seven as you find them. If they're personable and engaging, respond in kind. If they're aloof and detached, accept that perhaps that's what they choose to be, as individuals.
Be prepared for what will follow, because in the short-term, this individual will attempt to make life increasingly difficult for you. However, this will ultimately prove his downfall, as he gets more and more desperate and takes greater and greater risks to get under your skin. Consequently you MUST start to log these instances by writing them down in a small pocket diary which you own and keep in your personal possessions. Log every instance of bullying and intimidation, no matter how inconsequential. Date them and time them all. This will be your evidence, should it ever come to a tribunal; which it probably won't.
Remember that what you are dealing with here is a needy and manipulative child. They crave attention and will do almost anything to feed that craving. However, the more you feed it, the greater the craving. So by stopping and taking just a few of these simple steps, there will be some short-term pain, but you will find that your life in the office will become transformed and more importantly, this individual WILL get the message and either move on, or get taken out.