For the final boss level 81–85 depending on your gear and what level your arts are you could beat the game in as 79. For grinding go to the Buonis' interior and fight some unique monsters. At your current level you should stick to Prison island for grinding. when you get around level 79 Anu shore in Eryth sea is a good place to grind. I beat the game with Shulk, Reyn and Sharla.