You an train Pokemon in the grass
My favourite Pokemon would have to be castform, altaria, rayquaza, latias and latios, groudon (these are my party Pokemon and i love them to pieces). To get the best Pokemon you just have to train it up and take care of it eventually they will become great.
It is any pokemon if you train it right but I would say Groudon
you train them
train your pokemon
Yes, you can
Well to me, it depends what type your strongest with, water go for Kyogre, if and ground catch Groudon, and if you like dragon or flying type getraquaza.(apparently I don't think arceas is in Emerald).
Train them by defeating the Pokémon League over and over again.
Train a Chansey and will evolve.
Get a Golbat and train it.
ANYWHERE
There is no difference.