When you're fighting a boss in the game, say Shanks for example, He dodges one of your attacks, and then glows Blue for a few seconds. When that happens, they're Invincible for that time period. But, when they launch attacks, Try to hit A+B at the same time, to roll out of the way so you gain this protection for a brief moment. Most of the time, you'll avoid it, especially when they use their Final Attacks which mostly KO's a character. But what else you'll need is to Make your Characters HP Raise up above 1000-1500, Get a Ton of Hp Restoring Drinks and SP Restoring Drinks, and Flame Shells + Ice Shells + Poison Bombs. Use it in combinations, and a Boss will eventually go down. This is especially useful when Fighting Whitebeard, the hardest Boss in the game. And for the bosses the second time through when they all get increasingly harder.