First you have to get every pokemon in every game. Next you have
beat the frontier brains
Finishing the Battle Frontier in Platinum means obtaining all Silver and Gold prints handed out by defeating the Frontier Brains.
You unlock Silver Prints by beating the Frontier Brains in the Battle Frontier in Platinum.
the castle tower factory and arcade you have to beat 20 in each of those 42 in the other
you have to beat a number of people first battle castle = 21 battle factory = 21 battle hall = 50 battle tower = 21 battle arcade = 21
no
All of the Battle Frontier Brains will challenge you in their respective battle venue once you have beaten enough trainers. You can face Tycoon Brain Palmer in the battle tower after 20 battles and then again after 48 battles for example.
The Frontier Brains and the Champions.
You defeat the Frontier Brains in each of the facilities in the Battle Frontier.
go to battle frontier
The frontier brains are the heads of each battle facility in the battle frontier each of which are expert trainers if you ever reach them you can have the opportunity to win the frontier symbol from them. The frontier brains are: Brandon(Pyramid), Annabel(Tower), Lucy(Pike), Noland(Factory), Tucker(Dome), Spencer(Palace), Greta(Arena).
To get to the frontier brains, you have to win through a certain number of rounds in a row (eg. 21 battles in the battle factory, 4 tournaments in battle dome)