There is a spot later in the game where Batman will come upon a room that seemingly doesn't have a way out. You'll recognize it because it has three different security force field doors.
Before you know it, a Titan will break through one of the open walls, followed by some regular Joker goons. This is a great spot to get both the Freakshow Rodeo achievement and this 20-hit combo achievement. Simply dodge / batarang / hit the Titan until he is kneeling on one leg with stars around his head. You can them jump on his back and make him take wild swings at the goons. Each of his hits counts as 1 in a combo strike, and there are plenty of goons to get it VERY close to 20 before he bucks you off (usually between 16-18). Don't worry - there are plenty of goons close by after you're back on the ground that you can easily strike with a simple X attack or Y counter to keep that long chain going and your achievement will soon pop.