HyperBall

Hey, sorry if this is a stupid question. I was wondering if anyone knew the exact requirements to reveal the Aron cave. I have completed the grave stone riddle, the second gym, and retrieved the old doll (or whatever it’s called) and I would really like to have an Elekid on my team for my next play through. Although others have posted abt this they solved the issue themselves (something my dumb ass seems unable to do ) anyways thanks for reading and any help you’re willing to provide!