When you approach an enemy from behind, target him and when you are
close enough his head will flash in white, press square and you will
cut his throat and instantly kill him.

When using the hook, stop swinging and move down to an enemy, press
square and you will decapitate him.

When facing the first boss, keep about 5 steps away from him, target
him and run around, target him and press square to release the string
shuriken and that will inflict a lot of damage (you have to run to the
sides)
He will multiply into 2 and 3, be careful and hit the copies of
himself just one time, they will disappear immediately.

Continue attacking him and he will be dead in no time.