Heart gold/ soul silver- Go to Mt. Silver or victory Road, pokemon can range from level 40-50. Then when it evolves into a dragonite (Lv. 55) rematch Gym leaders, he will be underleveled, but he can hold his own against them with the right moves. Then when he is in his 70s, take the Elite 4, beware of lances dragons as well. Then in the 80s and 90s, take on red.
Fire red/leaf green-Cerulean cave/Victory Road
RSE-victory road/Sky tower
diamon/pearl/Platinum- Elite 4