Elite Four

What you want to do is take level 100 Pokemon in with you and then right down evry Pokemon you battle in the elite 4 then you find out there weaknesses and take them in don't give up because I had the same problem you are having but I just did what I said and they became a whole lot easier I hope this helped

Why are you asking this in Pokemon Sapphire about Diamond???

Try training your Pokemon to a good level. Also catch/use types that can deal massive damage to the opponent. Ex. Fire against Grass or Ice against Dragon.

They are saying that they have beaten the elite four in DIamond, but are finding it difficult to beat the elite four in Sapphire... That is because it is much easier to beat the E$ in DIamond and Pearl, due to the varying types of the Pokemon used.

It would help a lot to know what Pokemon you have on your team, Dark Water Fairy... That way we could see if it is a type match up problem.

Forget taking level 100 Pokemon with you, it is far easier to go in with the correct types of Pokemon odf a sufficient level. I have done this with Pokemon of around level 55 in my party and some lower. You also want to take in heaps of revival herbs (they fully revive your Pokemon instead of just reviving with only half their HP) Lots of ethers, elixirs, Leppa berries, in fact, as many restoratives as you can muster/afford, you will use them! Save between every battle, and reset it if you lose. That way you don't have to start from the start if, say you get to drake and almost win but get pipped at the post... You just reset and you go to just before you began the battle with drake, as opposed to back at the start!

But it is of the utmost importance that you use Pokemon that are strong against the Pokemon you are to face. Learn what Pokemon your opponent will use, and choose your team wisely. I suggest at least one ice type for the dragons you face, but I have used Flygon and Altaria most commonly against Sapphire's elite four and have had no problem.

A good fire type for the final Champion battle always did well against the steel types... But it depends on they types and when they come out. Going in with all level 40's will be a very tough battle, but it is doable. I would tend to try and have some strong Pokemon at least three of them over level 50. And I use them a lot, I also have 'sacrificial pokemon' that when a Pokemon I want to use passes out, I specifically bring in one of my sacrifices to use a revival herb on my stronger Pokemon and have that sacrificial Pokemon faint so I can replace it with my stronger Pokemon without losing a turn. (these can also be strong Pokemon, just not ones suited to that particular battle)

But really, what has always won it for me in the past is the fact that I tend to use Pokemon of dual types such as a water/electric type etc...

I suggest get some training... Up until level 60 is fine... There are Dragon, Dark, Ghost, Ice and the Champion... Ice works well with the Dragons. Dark you would need to determine each one's weaknesses. Ghosts... Psychic. And Ice use eletricity and fire. (Depends what) Champion use each's weaknesses.