You can get this without even trying actually, as you progress through the game and do combos you launch a lot more than twenty.
But if you must get it right away, use the X X X Y combo on heavier enemies and they will launch in the air. You can also use the War Hammer Construct, after it's given to you in Mission 1 and get it that way.
If you leave negative feedback, please comment why as so I can adjust this. Thanks.