Enemies are the things that walk around and hurt you. Plants, barrels, and items do jot count.
If you press X, a projectile will exit your character. You must time it so that this projectile collides with the hitbox of the enemy. Repeat this process a few times to deplete the enemy's HP and defeat it.
Now, the tricky part is counting to 150. I ran out of fingers pretty quickly, plus it's hard to count while holding the controller, so you may want to use a different method.