Get a Steelix that's a high level or a Torterra or any Pokemon that has dragon and ground moves. (so Garchomp). then use these sort of moves on his Pokemon and boom he goes down. (mind his electivire and those moving platforms)!
volkner
The 8th and last gym is in Sunyshore City.
sunnyshore city
go to the Pokemon league north of sunyshore
Defeat the 8th
The 8th gym leader in Pokemon Light Platinum is located in a city called Lauren. You can find the gym leader in the Lauren League Headquarters.
beat the 8th gym and then go up and there is a person from hoen and also gym leader will give it to you
Make sure you have a party of all water type, but if you want to OWN u use pellipers, Because they can Kill!
the 8th gym is closed because you have to beat all other 7 gyms and then it will be open p.s. the 8th gym leader is giovonni
After You Defeat All The Gyms Jasmine The 8th Gym Leader Of Johto Should Give It To You.
where can i find the second gym in Pokemon platinum
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.