I've been testing for an easier way to get mercy rules. Is it just RNG or can you actually use skill or manipulate the match to your advantage?

its all rng sadly

Ik, I'm five years late on this but I was watching a 99-0 baseball tas that made the opposing team keep committing errors. Would a human be able to replicate that and what is the game logic behind that?

Here's the TAS for reference:

Errors are completely RNG

Well, whether or not you get the mercy rule or not is in your control, and depends on which hits you get and the angle of your swing. If you are referring strictly to errors, there is no way to influence whether or not the opponent will commit an error against you.