You must pull off 20 attacks in succession without being hit yourself.
A good way to do this is to until the point you are fighting titans, just jump on the titans back, and mow over as many henchman of jokers as you can.
It takes time, but practice makes perfect.
You may also try this with a large group of joker henchmen, but if you plan to do this, do it earlier on, when theyre at their most vulnerable.