To defeat Ra's Al Goul at the end, make sure you defeat all of the ninjas that are around him; he will not die until all ninjas are dead. Also, when Ra's Al Goul is trying to attack you with the sword, it is best to use counter.
You can get everything in gallery of fear and two batmobile missions by completing the game at least once.
To unlock all Movies and Interviews, you must beat the game once on any difficulty.
When you beat the game, you unlock several things. It doesn't matter which difficulty. Unlockable How to Unlock
Unlock 2 Batmobile missions Beat the game once
Unlock 3 alternate Batsuits Beat the game once
Unlock all movies and interviews Beat the game once
Unlock everything in Gallery of Fear Beat the game once
To get the Caped Crusader Suit, Dark Knight Suit, and World's Greatest Detective Suit, you must complete the game once on any difficulty.
Complete the game on any difficulty.
When there is gas going on Hold R2 and move. Your health will not deteriorate.