I haven't gotten a lot, and I know they all are level 50, and my Pokemon right now are 2 level 56s a level 57, and three level 60s. If I do the Delta Episode, by the time I'm done, will my Pokemon be too powerful? To give you a point of reference, my weakest poke, using only it's weakest move, could've three shot regigigas, if, of course, I hadn't wanted to catch it.
You'll be okay! If you're worried, it wouldn't hurt to teach False Swipe to one of your Pokemon, or purposely use moves that the Pokemon you're batting resists so that it does less damage.
In the Rustboro City Pokemart, there will be a guy with a grey suit wearing glasses standing next to the shopping counter. Talk to him and he will give you TM54 False Swipe after a few sentences. False Swipe is a move that will never knock out a Pokemon; it'll leave it at 1 HP.
Hope this helps!
Well, I guess they will be, but there is a solution: you could train a False Swiper. My personal choice is Gallade, since he can learn not only False Swipe, but also Thunder Wave and Swords Dance. If you have the time to train a False Swiper, it can definitely be helpful (Except for Ghost types, like Giratina)
Hope I helped!!!