If you need a strong pokemon, and you dont have much time to train one alot, after you beat the Elite Four, you have access to the battle area, once you see all the pokemon (you dont have to catch them), you have access to the other two areas. Go to the resort area and go on the bridge with the lake with the tree in the middle in it. Fish with a super rod. You will most likely get high level Magikarps (the highest i ever caught was lvl 87), and catch one. Give it an Exp. Share and battle some people, preferably the ace trainers on route 227. It will only take a level to evolve it to Gyarados. I suggest stopping its evolution for the first few levels, so the pokemon doesnt need as many Exp points to level up. It may take 15 minutes or so, but in a little while of training it you should have a high level Gyarados. Or, a level 100 Gyarados.
Please give thumbs up and comment if i got something mistaken. Thanks.