Can be earned in a single freestyle session. Any song combination.
I recommend holding the button for the track you're inserting, and releasing on any downbeat. Easier for me than just pressing the button on beat.
In freestyle, you can change the tempo by pressing the button on the bottom left of the turntable console with three sliders on it. Lower tempo makes it easier to hit on the beat. Higher makes it harder, but makes getting the achievement faster. Your choice.