you should have a psychic type or a flying type such as staravia and if you are having trouble with her lucario use a fighting or fire type move(or ground) i used an action replay code to catch her lucario LOL
Kill them with Fire.
I have Pokemon platinum the 3rd gym leader always their your probaly just not lucky.
Defeat the 3rd Gym Leader, Wattson, in Mauville City.
Flash, I believe.
Go to the 3rd gym and battle the gym leader and then u found him
ground. He is an electric type.
I would get a rock or ground type and use dig or teach one of your Pokemon dig
You Go left and use rock smash to open up the tunnel
3rd gym
You use your strong(est) Pokemon to kill the Gym Leader's Pokemon.
Maylene (Veilstone City) uses Fighting-type pokemon. Use Flying or Psychic-type pokemon to defeat her. Wake (Pastoria City) uses Water-type pokemon. Use Grass or Electric-type pokemon to defeat him. I put Maylene and Wake on here cos you can battle both of them third.
Whitney ,she's at Goldenrod City.(Normal-type)After you defeat her she'll start crying.Talk to the nearest player and then talk to her and you'll get your badge.