You have one more obstacle before you make it to the Indigo Plateau and the Elite Four. The cave is really no big deal to get through, but the items in the cave are a big deal! Pick up as many as you can, then head out to Indigo Plateau.
Don't be too surprised when you rival shows up to challenge you right at the end of a long dungeon, mere steps away from a Pokémon Center. Be prepared for this fight when you head up to the exit. Your rival has six Pokémon now, and their levels are quite high. The new addition is Kadabra, a quick and powerful Psychic. A Dark-type Sneasel would be great against it, or any Pokémon with Dark-type moves.
Drop down for TM 26
You might want to pop into the Plateau and heal before you come back out, jump into the pit in the upper left corner of the third floor, and drop down to TM 26. This TM contains the Ground-type attack Earthquake, which is a must-have move.
|Rhyhorn, and its evolved form Rhydon if you are in Crystal. Evolved form Rhydon has amazingly high Attack and Defense scores. Their Speed and Special stats are fairly low, but the Special shouldn't matter since their best attacks are all Physical.
As is often the case in Gen 2, the time at which you face this Pokémon it is either level 35 in Gold/Silver or 32 in Crystal. This means that it will have forgotten its best starting move, Horn Attack. A Physical move that does 65 damage while being 100% accurate. It will instead have Stomp as its main Attack.
Can learn the move Horn Drill which is very inaccurate but will guarantee a knock out if it lands. At level 65, Rhydon can learn its first Ground-type's most powerful move, Earthquake, its only level 55 for Rhyhorn.