I'd always start with torchic, and then catch a taillow, zigzagoon and shroomish with effect spore. It should be easy then until you get to Tate&Liza. Then get a spheal from shoal cave and evolve it to a walrein on lvl42. This doesn't take too much time with the right training. Give it surf and get a sharpedo with crunch and a kecleon with shock wave to beat xatu. Once you have defeated Juan all Pokemon will obey you so you could trade over some decent ones for the elite4. Hope this helps!;)