Charge your sword by holding X, kill 2 enemies at once by releasing X to do a spin attack.
This can be done in the intro/training mission in the place where 2 tentacles pop up at once. They each take 3 hits to kill. When the both have only 1 hit left, run away a little bit to make them go back underground. when you come back, they may be close enough together that one spin attack will kill them both after the smack the ground and are stunned for a few seconds.
If they're not right next together, run away and try again.