it depends on the pokemon. basically get them to higher levels and don't be afraid to use items. i beat it with a servine lvl19 and a tympole lvl 20 with 2 cannon foder pokemon for when i wanted to heal.
battle wild pokemon until yours is lvl18+
Its best if you use fighting type pokemon. black- Sawk would be your best to use to beat the gym white- Throh would be your best to use to beat the gym.
you die
Find ALL the trainers beat them all, and then fight the Gym Leader at the front.
Have a water ice or grass pokemon.
beat misty in second gym
hm01 cut
After beating all the Gym Leaders, complete your pokedex.
beat the gym leader
You buy them when you beat your 4th gym
you have to beat gym leaders
good question, well i got white and there like the same thing. after you see N in the place after you beat the seneth gym. you fly back to the second gym and the gym leader there will give it to you.