I think that you've asked the same question 2 times and already received a good summary of what discovery is about and how important it is.
No attorney can give you a brief answer to your question of "how to reach the best result to prove a case." That is what litigation and trial lawyering are all about. It requires a detailed investigation and understanding of the facts. Likewise, it requires a detailed understanding of the applicable law including the law of evidence. And it requires understanding the art of persuasion. These are things that lawyers to go law school and then train and gain experience for years to master.
I assume that you are the plaintiff. If you have a meritorious case, many lawyers are likely to consider representing you, and in discrimination cases - if you prevail - your attorney has the opportunity to force the defendant to pay at the prevailing hourly rate for the time spent representing you on the case. I suggest you consult with an attorney.
From the way your question is framed, I think you need to retain a lawyer. The discovery process is a means of obtaining information relevant to the competing claims. It takes several different forms. it is rule bound and contains multiple traps for the unwary. As to what information emerging from the discovery process may be critical to your case, only a professional intimately familiar with case would know. Given the level of knowledge that your question reveals, you are likely headed to dismissal of your claim or a resounding defeat. Professional help is essential to maximize whatever merit your allegations may have.
Respectfully, there have been books written on this subject. Lawyers spend years of their lives on this issue. You either need to retain local counsel, or spend many nights and weekends in a library. I recommend the former.