As one of the best, you need to man up and defend your base from enemies. These guys would eventually know that you are not the guy to mess up with. The idea in this game is to simply kill all the enemies. Don't worry as most of the time, you would have some backup to call upon. You can also make money from killing enemies and you can use it to buy better weapons. The game would be harder and harder as you make some progress, but don't worry about it as that is not hard if you are really good.