In Blackthorn City.
Clair is in Blackthorn City, you can battle her straightaway
the potion is in the pharmacy in cianwood city
just defeat blue, the 8th gym leader
If you are talking about the 8th gym leader in Johto, you go to New Bark Town, surf east, and go to the Pokemon League. If it's the 8th gym leader in Kanto, go to Saffron City to receive a Hoenn starter from Steven Stone.
u will find him on Cinnabar Island near the northern side. But he won't battle you until you beat Blaine over at Seafoam Islands. hen go talk to him and he will return to his gym where you can then battle him.
if you you meant after you beat her,go to the dragons den on the back of the gym.you need surf and whirlpool.you have to past the test the master tell you(clairs grandfather).
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.
use a garchomp that you could trade from a sinnoh game (platinum, diamond, or pearl) i used a lvl 53 and kicked they're butts.
In Blckthorn City
In celadon city ^Technically, that is true for the first generation of Gold, Crystal, and Silver. But in heart gold and soul silver, the fourth gym leader in Kanto is Misty (cascade badge) if she is not in her gym in Cerulean City, then you can find her up on Route 25. ~Kt
8th Gym Leader: Claire8th GYM Leader is Claire of Blackthorn City. She uses dragon Pokemon. Yes indeed Claire is the 8th gym leader in gold/silver/crystal and she uses dragon type Pokemon just like lance
In the lighthouse in Sunyshore city