Unfairly assessed low performance, can lead to dismissal, what to do?

Need legal advise how to handle this best.

After more than 12yrs of being considered among top performers in the company with several promotions in between, I had a new boss that obviously did not like me.

The recent performance review by my manager (who is based overseas) for 2013, I was given "below target" rating, but without supporting documented evidence. In our company, this would mean I am rated at the bottom of my peers. I am sure that I am not a low performer as my boss can't show any particular proof or evidence, nor a direct feedback from teammates and other superiors who can see my daily performance. This is the very first time I was given a "below target" performance assessment and that I need to improve or else I can be terminated for just cause.

If I resign or get terminated for authorised cause (retrenched), the money I will get is quite significant. If they terminate me for just cause, I may get nothing.

So need expert legal opinion how to handle this.
1. How to legally contest the assessment of my manager who sits overseas?
2. Does my previous 12years of above average performance have any bearing in this argument?
3. What case can I raise in this case? Against my manager, and against my company?
4. What should the company legally do if they want me out? So I know what to watch out for.

Thanks a lot.

So need expert legal opinion how to handle this.
1. How to legally contest the assessment of my manager who sits overseas?
--> check your rules and guidelines how to contest such assessments. If there is none probably sending an email to you supervisors will be okay.

2. Does my previous 12years of above average performance have any bearing in this argument?
--> Standards change from time to time. The law has what we call "management prerogative" so standards may change. This 12 yrs of previous above average rating can help but rating does not mean that you will always perform above average.

3. What case can I raise in this case? Against my manager, and against my company?
--> hard to prove that you are being singled out. Wait it out. Request for another manager. or try to perform better. (easier said than done)

4. What should the company legally do if they want me out? So I know what to watch out for.
--> NTE (notice to explain) you will be asked to explain your low performance.

Thanks @udmlaw.

Would appreciate if I can get sustained help and guidance from a certified labor lawyer to pursue this case. My company is a blue chip multinational, and money at stake is quite significant.

Goes without saying that I'm willing to give monetary reward after winning this case.

Pls contact me via PM. Thanks.