i'd invest in a gible, it evolves into garchomp, a powerful attack sweeper, you can find them in the waward cave a(which is pretty early in the game)--if you can breed it to learn outrage, you'll have a really strong pokemon
also, ralts is a good choice--it can become either a good sp atk or atk sweeper. You can find it on the southern parts of routes 203+204 (you could also breed this one to learn some good moves like memento and destiny bond) im not sure if you need the national dex to catch ralts, but i dont think you need it.
Thanks man. Ralts is great I love using it in Emerald