Stick To Training One Single PokemonThere are really only two ways of training your Pokemon in Pokemon GO, just like in the Mainstream Games, either by committing all of your time, Stardust and Candies into one Pokemon or in a balanced distribution among a number of your Pokemon. Both ways have pros and cons to them, but we're going to focus on the former, the ones for training a single Pokemon.
When putting all you have into raising just one Pokemon, you actually increase your chances of one day being a Gym Leader, and one that won't easily be defeated. No one can easily defeat a Pokemon at Level 3000 or something unworldly, and that's something you could have with this way of training, especially if you do so with a Pokemon Evolution Chain that is easy to catch in your given geographical area and has a higher Maximum Combat Level. Plus, all your friends and players alike with be jealous when they feast their eyes on such a strong Pokemon.
But there are also a number of set backs for this way of training, the first of which being the extreme cost of training that one Pokemon. You'll have to spend more Stardust and Candies on this one Pokemon, compared to if you did a more balanced approach. You'll also be in trouble at Gyms, since it will really be the only Pokemon that can stand a chance. If it falls, you'll probably lose that Gym. And let's not forget the fact that if you do win over a gym, you will have no other viable options in your Party unless you catch some rare Pokemon while putting all your effort into this one. Your Pokedex will also most likely take a hit too, since you likely won't be evolving many Pokemon with all your resources going into this one Pokemon.
You could be the King of Gym Leaders by using this path, but a more balanced effort towards all your Pokemon is more always recommended with a game like Pokemon GO. Better save this technique for the regular Pokemon Games by Nintendo.