there is a short guide on the AI system, for silent line, on gamefaqs.
I've got a few simple tricks that tend to work.
1. simple > complex. Yes, you can make an AI unit which fights like and surpasses rank 1 (Infinity) but the time it takes to train the AI will result in all your friends leaving you. A simple Sawa + optionals set up can make a truly deadly AI unit and is much easier for the AI to pick up and learn. Ditto on dual weld set ups.
2. Humans love machine guns, but they are tricky for the AI to learn. They tend to fire them in bursts rather than holding the trigger. It takes a lot of time to get a proper MG AI unit.
3. The AI unit picks up your good and bad habits. If you can stand it, fight easy opponents in the arena to "level up" your AI. Keep in mind that the AI does not see a difference between ranks 20 and 1. If you fight rank 20 over and over and teach your AI to constantly get behind its target and to not be in the locked on status the AI unit will employ those same tactics against much more difficult opponents with the same consistency. On the flip side, if you miss a lot and are a red zone addict your AI will be too.
Don't be afraid to give your AI unit really cheap parts too. the CROW stealth extensions and OP-INTENSIFY come to mind. These AI units are not taking part in a tournament, or PvP, so there isn't a reason to limit their part choices.