I need really strong pokemon to defeat the Elaine 4, which Pokemon do ..

You should take these pokemon when you fight elite 4 :

Altaria/salamence
groudon/kyogre
Gardeviour/Alakazam
Wailord/gyarados
Mudkip/blaziken (sceptile suck against elite 4)
Hariyama/machamp

I bring a lot of Leppa Berries, and my Kyogre, Groudon, Rayquaza, and my Tropius to beat them.

My Kyogre Knows: Water Spout, Sheer Cold, Thunder, and Blizzard.

My Groudon Knows: Eruption, Fissure, Fire Blast, and Solarbeam.

My Rayquaza knows:Blizzard, Thunder, Extremespeed, and Hyper Beam.

My Tropius knows Fly, because if you dont have a flying Pokemon, it can be hard to get somewhere when you're stuck in Littleroot! {all I use is my Kyogre!]the Empty slot you should put in a pokemon that is holding Exp. Share!

Bring in a psychic pokemon (prefferably with a fighting type as well to defend against sydney/pheobe) that knows psychic, ice beam, thunderbolt (tms 13,24, & 35 at the game corner) and recover
(learned naturally) (these are effective/super effective against all pokemon but electric/steel hybrids) which can be used against any of the elite 4, a fighting pokemon with fighting moves and a non-psychic subtype such as bug (this helps against the grass types) (sydney), a normal type with at least one dark move and a combo of other strong moves (pheobe), a fighting type with fighting moves and ones strong against water and ice (preferably not a hybrid) (glacia), a pure ice type with strong ice moves (drake), and a fire/ground type with strong fire and ground moves and a superpowerful normal or fire move for claydoll (steven)

Salamence about lvl 55, Blaziken lvl 55 (at least), Groudon lvl 55 (at least)
You might want a Altaria too at lvl 52 at least because Drake the dragon master is CRAZY! Oh-and here's my advice-get a ghost Pokemon at at least lvl 52 so you can beat up Pheobe, the ghost girl or some other nickname. When you verse Steven you should put Groudon in front and use Earthquake on all of them (except Scarmory-use Fire Blast) For Cradily use a fighting move (I suggest Blaziken)