Challenge implies that it is difficult every time you play it. For gold/silver, Geodude stomps the first four gyms, Abra line stomps the whole game, Espeon destroys 3/4 members of the Elite 4 by itself, Lapras destroys the Champion, and Cyndaquil line destroys everything in between.
For Red/Blue, any semi-fast Pokemon with slash or a move that has a high critical hit ratio destroys the game. Persian with Slash and Bubblebeam destroys most of the game.
It really isn't hard. At least the trainers in the new games can cover their weaknesses and can competently use status moves.