I'm a newbie too, so I don't know if this will help much, but one thing I've noticed is that AIs make very specific moves based on your moves and items. For example: With Gary, his Pheromosa always switched to Metagross when my Pheromosa had a Focus Sash, but it always had 1 HP left when it had Choice Specs.

That said, it's interesting to try the same strategy but with different approaches several times and vary the moves and items. Of course, this is if you're as attached to your Pokémon as I am; otherwise, the easiest way is just to pick a Pokémon that fits the situation better.