Good question! If your Pokemon are at a level of 45-50, then listen up. After you beat the Elite 4, go to Pacifidlog town. Surf east until you find an and small kind of maze. If you see it, go left, up, right, up, then you find a cave, with no Pokemon. Go up until you reach the exit, then you arrive at Sky Pillar, where Rayquaza is found. Save your data at least before 2 steps at the entrance. Done? Read below for more. Of course, train with an Exp. Share for better results.
For Pokemon at the Level of 40-50:
Stay and train at level one (means floor one).
For Pokemon at the level of 50-55:
Stay and train at level 3 or above (means floor 3 or above).
Keep on training, and you'll have strong Pokemon at no time.
Not to mention, for Pokemon a bit weaker, around levels 30-40, train near Victory Road, and for Pokemon 50-60, training at the Elite Four can also be great. Hope I helped.