I found this the easiest to do straight after you destroy the third pump (after killer croc, before ivy). You get trapped in a room with a titan henchmen and many thugs and got it in my first riding sequence. This was before the reinforcements where there was even more thugs and thus making it even easier to get.
Remember, to ride a titan henchmen you have to throw a batarang whilst it is sprinting at you and then knock him down after he is stunned from hitting the wall, then you will jump on and ride him.
Hope this helps!