Tears of a Blue Demon

The Blue Demon is a brutal optional boss during main mission 2, "Infiltrating the Black Ship" - you can flee it first time round, and most people do. Come back through Mission Replay once your build's strong: learn its telegraphs, block and deflect to break its Ki, then punish the stagger. It's entirely optional, so no shame in leaving it until you're properly kitted out - one of the harder fights in the game.