Hold the target button to lock-on to an enemy (default: R1). Then move the joystick in the direction away from the enemy you have targeted and press the melee button (default triangle/Y). This will launch the enemy. If you do this and hold the melee button instead of tapping it, you'll jump up with the launched enemy to follow with air juggles.
As a classic DMC player, I don't see the reason for a forced lock on in order for the launcher to work... smh I have a pet-peeve for games that force lock-on. I forgot if it was the case at one point.