Here, I'll post a small guide. It will show some ways to beat the leaders.
Roxane
If you choosed Treeko or Mudkip as your starter, then this should be a piece of cake. If you choose Torchic as your starter, then I recomend that you evolve it to Combusken and teach it Double Kick, so it can have a super Effective move agganst the Rock types.
Brawly
Combusken will do pretty good if you taught it Peck. Grovyle and Marshstomp will be average here. If your still doubtful about your Starter's ability in this gym, then you can catch a Taillow/Wingull/Abra for this gym.
Wattson
Marshtomp owns this gym, and Combusken does very well here as well. Grovyle will have some problems will Magnemite and Magneton Though, so I suggest that you capture a Geodude or Aron for this gym if you choose Treeko as your starter.
Flannery
Once again, Marshstomp Owns this gym. Combusken will have some problems, and sending out Grovyle is suicide. So if you choosed Torchic or Treecko as your starter, then I suggest that you get Rock, Ground, or Water type. Here are some possible Canidates. Geodude/Graveler, Marill/Azumarill, Sandshrew/Sandslash, Gyarados. Lunatone (Sapphire), Solrock (Ruby only).
Norman (Dad)
Combusken/Blaziken can make Norman's Pokemon go crying to their mommys. Mashstomp/Swampert and Grovyle/Sceptile do all right here. You need to watch out for Slakings' Focus Punch however.
Winnona
All three starters don't do very well here, so I reccomend getting a Electric (Manectric comes to mind;310; ) or Rock type. All though Blaziken COULD be used to knock out Skarmory...
Liza & Tate
It's Solrock you need to worry about. It Knows Solar Beam, Flametrower, and I beleive Rock Tomb. Lunatone Knows psyhic, So Swampert, Blaziken, and Secptile won't do Very well. I suggest getting Absol and Train it until it can Withstand a few Flamethrowers or a couple of Solar Beams.
Wallace
Secptile finnaly gets a chance to shine. It's Leaf Blade can put a major dent in Wallace's Pokemon. You'll need to watch out for Miltonic's Ice Beam however...
Sindey
Blaziken OWNS this match. If you choosen a different starter though, Then Machoke/Machamp, Medicham, and Heracross do great here to.
Pheobe
The starters won't help you much here. The Absol that you used aganst Liza & Tate could help here, Even a Cacturne with Faint Attack will do well.
Glacia
Blaziken will do well here, just watch out for Surf. Sceptile could do well here, Just watch out for Blizzard, and Swampert does average in this round.
Drake
A Swampert with Ice Beam will work wonders here. If you have Blaziken or Sceptile, then get a good Dragon (Salamence, Flygon) or Ice type (Glaile).
Steven
Blaziken OWNS this match. Period. Swampert with Earthquake can cream Steven as well. Don't count on Sceptile much though.
There you go, a gym leader guide that should come in handy. I hope I've helped you.