To unlock hard mode do you have to beat the game, only to delete the file to play on hard.
Thanks for the quick answers.
You have to beat the Elite Four to unlock the Hard Mode Key, which you can start using for post game, but if you restart then you just lock it again. The only way to use the key from the beginning is to have a second game that receives the key from a Black 2 version that's already beaten the Elite Four and it can only be received through IR. You also can't transfer the key to another game, start over, and have that other game unlock it for you. Only games that have had their Elite Four beaten and have unlocked the key in that way can unlock it for others.
I'll beat the game, trsfer my Monsters, then play again on hard then.
Deleting your save to start again will delete the challenge mode key too
You're saying its impossible to play per e4 on hard mode then.
No, it's possible, you just need two copies of the game, and one of them has to be Black 2, as it is the one with the Challenge Mode key (or know someone with a post-E4 copy of Black 2). White 2 has Easy Mode. Just start a new game in White 2, transfer the key from Black 2, and put White 2 in Challenge Mode as soon as you're allowed.