First of all, it is not called "medico lego"...the correct term is "medico-legal" case which means a case arising in the context of doctor-patient relationship.
Just because a patinet died or had serious side effect causing injury to a patient does not mean you have winnable case.
In order to win a medico-legal case. you ahve to prove the following conditions:
(1) That there was a doctor-patient relationship. (2) That the injury/death was directly caused by what a doctor failing to do something which he should have done. OR any other average qualified doctor would have done in the matter.
(3) A doctor did something so wrong that it was directly resulted in death or injury to the patient.
Please note that the patinet's side have to prove clearly that the injury/death was caused based on above facts. Most of the time, there is lot of emotions and anger but there is not enough evidence to win the case. It is common for people to get injured or die during medical treatment because it is part of the process of disease and treatment.
Unless you can prove that injury or death was direct result of negligence of doctor, you can not win in court of law. You will need medical doctors who are experts to research and provide opinion in the court about cause and whether there was negligence.
To prove negligence of doctor is NOT an easy matter. But, if you truly have a case, it is possible to win.