just go and beat it!
you need a fire or flying type Pokemon to beat that gym b/c if you started out with piplup or turtwig you might have trouble. you can catch ponyta or starly and train them to at least lv 17 make sure your ponyta knows ember it makes life easy.
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go to hearthome city
Train it so it can have resistants
where can i find the second gym in Pokemon platinum
The second gym is located in Eterna City and uses Grass-type pokemon.
On the second floor on victory road there is a veteran clayten and his second pokemon is a level 47 licky licky