well this is the cave and there is a good photo if you can get it near the end of the level, you start off travelling downwards and a few zubat will fly up and out [o], you will see some grimer take a photo of both of them, then you will see some bulbasaurs, hit them with pester balls, they will change into ditto [o], looking behind you wil see a grimer on a ledge, hit it with pester balls, it wil evolve into muk [o], any way carry on and go through the opening ad hit the koffing chasing a jigglypuff with a pester ball, and go through, do this to the other 2 if you can, and you'll see a weepinbell around a pool of water[o], hit it and it will evolve into a victreebell[o], proceed through the hole and try to hit the zubat with a pester ball or apple and he will drop the pikachu, he will sprout balloons and drift to the ground Pikachu on balloons and move on you will see 2 jynx around an egg [o], play the pokeflute if u have it (if not don't worry), and you will see an articuno pop out of the egg [o], if you rescued the pikachu before, turn back after the opening, you'll see pikachu on top of articuno pikachu on articuno, and look to your right you'll see the jigglypuffs you rescued before [o], and finish the level.
if you have enough points you can proceed to the valley