Playing the game with an underleveled team

So, I usually like the challenge presented by battling leaders/powerful trainers with an underleveled team; it just increases the satisfaction when I win. This is how things have been going:

Falkner: Won second try, 9 Totodile, 8 Gastly
Bugsy: Won second try, 10 Zubat, 12 Totodile, 12 Gastly
Whitney: Won first try, 14 Zubat, 13 Gastly, 14 Totodile,
Morty: Won second try, 17 Zubat, 17 Growlithe, 18 Dratini, 18 Gastly, 18 Croconaw, 8 Kangaskhan (lol)
Chuck: Won second try, 24 Crobat, 25 Croconaw, 23 Dratini, 24 Haunter, 24 Growlithe
Jasmine: Won first try, 25 Crobat, 24 Croconaw, 24 Dratini, 25 Haunter, 24 Growlithe
Price: Won first try, 27 Crobat, 28 Croconaw, 26 Dratini, 28 Gengar, 26 Growlithe
Clair: Won first try, 33 Crobat, 33 Feraligatr, 34 Dragonair, 36 Gengar, 35 Arcanine
Kimono Girls: Won third try (fuckin' Jolteon): 35 Crobat, 37 Feraligatr, 35 Dragonair, 38 Gengar, 37 Arcanine

Now, this is how the E4 on the first try went:

Will: Won first try, Feraligatr, Gengar, Dragonair KOed: 37 Crobat, 38 Feraligatr, 38 Dragonair, 39 Gengar, 37 Arcanine
Koga: Lost, KOed Ariados and Venomoth, revived entire team beforehand

Holy crap the E4 is hard ajlsdkfjalsekfjlawekj I have to buff up my Pokemon, if I lost at Koga with a full team, I don't know how hard Lance will be...so I'm getting my entire team to like 43 or something. Then I can gloat that I beat Lance with a semi-underleveled team. :3

So how about you? Do you guys just level up one Pokemon and go for it or what?