you can beat practicaly anything with torchic in the game (as long as it's higher levels) since it learns awsome moves as a 3rd evo poke... swampert too (gota TEACH it to it, but not too late) treeco's the one where u get a nice sweet smooth start and gets creamed in the league -_-" ... (as for your "battles cannot be won without 'insert poke here'" situations... i'd have to say, DONT bring treeco to see steven ... my sceptiled got SPIKED to death... embarasin, but i didn't expect him to be there...
*edit*
sorry i didn't answer your question like i wanted to, heres a quick info feed:
1st gym: rock pokes, use combuskin's fighting techs (double kick)
2nt gym: fighting pokes, use combuskin's fire techs (ember)
3rd gym: elec pokes, combus/blazekin can take down magnet family with fighting and the rest with anything you wish
4th gym: fire pokes, this one's tough; see side note
5th gum: daddy's normal gym, use fighting techs (i accidentali burnt slaking coz i thot it was a good idea since it had no sp. defense... had my **** FACADED BAD)
6th gym: flying pokes, bad for blaze; see side note
7th gym: psychic pokes, bad for blaze but the boss battle are rocks... also see side note
8th gym: water gym... ... you could TRY, but it's a water gym...
side note: catch a lotad in your hometown to train it up to become the awsome ludicolo (or if you CAN, try to find a seedot which is EXTREMELY blardi rare... i waited for its out break =S) lotad's awsome dual type makes it resistant to all the major battles in the game (including the pokemon league ones; which blaze would have no trouble handling single handedly) lombre's stabbed grass moves can help you through the 3rd and 4th gym faster (although lombre hasn't a significant advantage over 3rd gym, nothing's resistant against water in the third gym =P) the 4th gym leader uses fire pokes that knows ground moves (both types don't harm lombre that much) 6th gym uses flyers, they can do a number on lombre unless it had some ice techs on lomby (otherwise just catch a electrike and evolve it into manectric right before the weather station and pwn it) 7th is psychic, but the boss fight = rock (double battle) so having nuzleaf/lombre would help ALOT (by now u would've found leaf stones to turn them into awsome shiftry and ludicolos) and finaly, the last gym take advantage of grass techs...
there! done! XD once you get to lillycove you can buy all the techs you need XD