Be sure to build up your Echo Meter during each battle so that you can unleash your special attacks with more damage. You build the meter by attacking enemies with your regular attacks, which will build it up to 4 hits, 8 hits, 12 hits, 16 hits, 24 hits, and 32 hits (the latter being the highest). Unleashing special attacks at that plateau will result in the most powerful attacks your characters can use, but be sure to use them in the present battle, because the meter, even if unused, will be reset before the next battle.
If you're in need of money, remember that you can take pictures with Beat's special skills and sell them to vendors. Pictures of enemies and especially bosses are valuable and can net you lots of cash.
To unlock Encore Mode, which is Eternal Sonata's New Game+, you must beat the game once through. After you beat the game, watch the ending and save your game. Then, start a new game from your old save file.