You need to acquire 10 ribbons, on a Pokemon, and have that one in the lead position. Enter the Syndicate and the lady will check you. If you have 10, she will let you in.
You have to see every pokemon in the pokedex. Then go talk to Prof. Rowan in Sandgem. He will give you the National Dex. The people blocking your path should have moved when you go there now.
If I am not mistaken it is a level 100 magikarp
I havent found anything higher than a level 89 though, but the only thing you can catch in the pond is magikarp so... yea