How difficult is it for anyone with an arrest record to be granted a successful outcome if the petition for Factual Innocence is filed and if the case was dismissed via PC 1385 without any conviction,fine or probation etc?
That's like asking how long is a string? It's as long as it needs to be. For a declaration of factual innocence a great deal depends on what is in the file (including police reports), what you were charges with and why the case was dismissed. 1385 covers a lot of reasons the DA cannot go forward. If the case was dismissed because your abused girlfriend decided not to testify against you your not going to win your petition.
This will be particular to the facts of your case. You have to prove that the facts are such that there is no reason to believe you committed the crime. In some cases this would be very easy, in other cases it would be impossible.