Oh dear god, the Elite Four was hard.
Train your pokemon in Victory Road is the only thing I can come up with..
Also, if you got the Larvesta from that Hiker and raised it to a pretty good level, it's helpful too

spend all money.

just fight them again and again for exp and money

as long as you can beat just one. it's good exp

The thing I did is that everyday you can return to the city with the big and little stadiums (where the football, soccer, tennis, and baseball games are held (it's all next door to the Pokemon musical theater)) and fight different trainers. Their level goes up as you progress through the game and there are more to challenge as well. Everyday only in the evening their is also the cruise ship but this option isn't too reliable because some people on board are not trainers and instead talk so much that your cruise will automatically end. Also there is no healing while on board (I think I have yet to actually find one). -Also a slight warning! Expect the trainers you face after beating the game to be seriously difficult so I would really advise you to train at least 2 to 3 strong (lvl 59 to at least 63) Pokemon unless you plan to trade from older games to help out because believe me it gets a lot harder then you would expect! The training methods I listed will also grow to the same difficulty as well so be careful!-