Think of replacing one of the Blaziken, for type variety maybe.
Here's the lowdown on the E4:
Sidney uses Dark-types, weak to only Fighting and Bug. For his Shiftry, Sharpedo, and Cacturne, you can take advantage of the second type and nail it (Fire for Shiftry etc.). Don't use Fighting types against his Shiftry or Absol; they'll get owned.
Phoebe uses Ghost-types, weak to Dark and Ghost moves, both of which very rare, in addition they're immune to Normal and Fighting. These customers have somewhat high Defense, so a good strategy is just to chip away at 'em. The Sableye however has no weakness and immune to Psychic types, but not as high a Defense.
Glacia is an Ice-type user. However, 3 of her Pokemon are also half-Water, so against them you'll have to use Electric (not to mention with their Thick Fat, Fire moves are horrible). The other two, Glalies, are pure Ice, so Fire works there. (Ice is also weak to Fighting, so one Blaziken could give them all the go-around).
Now it's time to fight Dragons. Your best bet is to use Ice type moves that aren't used by an Ice-type, as he has moves like Flamethrower. Only Shelgon has a x2 weakness to Ice (the others have x4). If you have a Kyogre or Kingdra with Ice Beam, they'll get pummeled.
*In Emerald, one of his Flygon is replaced with Kingdra, whose only weakness is Dragon. Take it down with a heavy hit.
#5 Steven (RS)
Steven has a team of Ground, Rock, and Steel types, so to win you'll need to switch around a lot.
Skarmory - Fire and Electric work great here, as its SpDef is low.
Aggron - 4x weakness to Fighting, and 2x weakness to Water. Use caution when using Groudon or Kyogre against it - it has Thunder and Solarbeam.
Claydol - It has Reflect and Light Screen, so use Dark, Ice, or Water to KO it ASAP.
Cradily - Ice or Fighting is your best bet. But beware, it has Sludge Bomb, Confuse Ray, and Giga Drain. If you have several Pokemon with a type advantage, do not hesitate to swap them out if Poison or Confuse appears.
Armaldo - No fair, he gets two fossils. Water is its only weakness, and it can survive a lot of hits. Additionally it has strong moves.
Metagross - This is his strongest. Use a Fire or Ground move, before he hits you with Earthquake or Psychic. Another way is to use Water - he's not resistant to it.
*In Emerald, you'll fight Steven in Meteor Falls. His Pokemon are in the high 70s.
#5 Wallace (E)
This guy has Water types, but they're very hard to get at. Be prepared for repetitive switching.
Whiscash - 4x weakness to Grass. Hit him heavy with Sceptile or something.
Tentacruel - Electric or Psychic can get this guy, but it may take a while to bite away at. Don't give up.
Gyarados - You'll need an Electric to take down this guy, as Intimidate cuts attack, and thusly cuts Rock moves. (Not to worry, a Thunderbolt is a sure kill.)
Ludicolo - Its only weakness is Flying. Not to mention he'll use Double Team several times so he's impossible to hit. Then - a lot of Surfing. Faint it quick.
Wailord - This is the first he'll send out. Hit it with Grass or Electric, as it's a pure Water type.
Milotic - Beware the Ice Beam if you deploy Grass. Use Electric if you have it, or Milotic will be a living nightmare.
A full list of their moves is here:
Happy battling, and good luck!