If my memory is correct, to beat the Elite Four in FireRed, I just leveled up my Pokemon by fighting the Elite Four. It both gave me a chance to level up my Pokemon to prepare for the fight in which I actually plan to beat them, and also gives you a chance to beat them while training. (Which I think I did.) The only downside is that you could end up losing a lot of money, so if that's a major concern, you're better off just fighting Pokemon in Victory Road. Also, you could save money doing it by not using any healing/reviving items and saving them so you don't have to buy more for the battles that come after you're done training. However, I'd suggest that you start using them if you unexpectedly do really well.