One of the things about AI is that it tends to focus entirely on one thing, and forget everything else.  

The AI builds hundreds of slow moving fighters.  They would move halfway across the map, spending dozens or even hundreds of turns to attack one enemy.  But if you bribe them to attack someone else in a different area of the galaxy, the AI ships would then migrate halfway across the galaxy to attack the new target, ignoring the first enemy.  

Through this method you can "kill" hundreds of AI fighters at no cost because they're just moving in circles around the map doing nothing.  

If you can get Diplomatic Translators and get a little bit ahead in tech, there's all sorts of funny things you can do to the AI even at Obscene.