Like I am stuck at the Elite Four and Steven SPECIALLY, I have a Lv.58..

Put Blaziken up one more level to learn its best move sky uppercut, and take off cut. Evolve Kadabra too by trading it.

You team is fine! You should have no problem beating steven. Just train blaziken one more lv. and teach it sky uppercut instead of double kick. But you should have a lv. 50 or higher water pokemon that knows surf and ice beam. I suggest pelipper.

Here's an obvious one: GET MORE POKEMON!

Teach Blaziken, Blaze Kick, erase Slash

Teach Groudon, Eruption or Solarbeam or Fissure, erase Slash

Teach Kadabra, Hyper Beam, it's powerful and it's a lot better than Teleport; Teleport is useless
Note: Buy escape ropes in place of Teleport.

Your Pokemon should be at level 60 or above for 2 main Pokemon and 55 and above for minor/unimportant Pokemon.

You should catch Bagon (at the back room of Meteor Falls) evolve into Shelgon at level 30 something then into Salamence at level 50: Teach it Fire Blast, Fly and 2 other moves.

Catch Trapinch evolve it to Vibrava (you may also evolve Vibrava to Flygon.) Obviously teach it dragon moves and ground moves preferably Earthquake (if possible)\\

Catch a water pokemon like Wailmer.
Wailmer will evolve into Wailord at level 40. DON'T LET IT EVOLVE. AT LEVEL 41 WAILMER LEARNS WATER SPOUT. Wailord learns it at level 46 but it's usefull.

NOTE: NOT ALL POKEMON NEED TO FIT MY LEVEL 55 STANDARD, I RAISE THEM WITH RARE CANDIES I GET OUT OF AN AR CARTRIDGE.